iio: lpc32xx_adc: fix warnings caused by enabling unprepared clock

iio: lpc32xx_adc: fix warnings caused by enabling unprepared clock

If common clock framework is configured, the driver generates a warning,
which is fixed by this change:

    root@devkit3250:~# cat /sys/bus/iio/devices/iio\:device0/in_voltage0_raw
    ------------[ cut here ]------------
    WARNING: CPU: 0 PID: 724 at drivers/clk/clk.c:727 clk_core_enable+0x2c/0xa4()
    Modules linked in: sc16is7xx snd_soc_uda1380
    CPU: 0 PID: 724 Comm: cat Not tainted 4.3.0-rc2+ #198
    Hardware name: LPC32XX SoC (Flattened Device Tree)
    Backtrace:
    [<>] (dump_backtrace) from [<>] (show_stack+0x18/0x1c)
    [<>] (show_stack) from [<>] (dump_stack+0x20/0x28)
    [<>] (dump_stack) from [<>] (warn_slowpath_common+0x90/0xb8)
    [<>] (warn_slowpath_common) from [<>] (warn_slowpath_null+0x24/0x2c)
    [<>] (warn_slowpath_null) from [<>] (clk_core_enable+0x2c/0xa4)
    [<>] (clk_core_enable) from [<>] (clk_enable+0x24/0x38)
    [<>] (clk_enable) from [<>] (lpc32xx_read_raw+0x38/0x80)
    [<>] (lpc32xx_read_raw) from [<>] (iio_read_channel_info+0x70/0x94)
    [<>] (iio_read_channel_info) from [<>] (dev_attr_show+0x28/0x4c)
    [<>] (dev_attr_show) from [<>] (sysfs_kf_seq_show+0x8c/0xf0)
    [<>] (sysfs_kf_seq_show) from [<>] (kernfs_seq_show+0x2c/0x30)
    [<>] (kernfs_seq_show) from [<>] (seq_read+0x1c8/0x440)
    [<>] (seq_read) from [<>] (kernfs_fop_read+0x38/0x170)
    [<>] (kernfs_fop_read) from [<>] (do_readv_writev+0x16c/0x238)
    [<>] (do_readv_writev) from [<>] (vfs_readv+0x50/0x58)
    [<>] (vfs_readv) from [<>] (default_file_splice_read+0x1a4/0x308)
    [<>] (default_file_splice_read) from [<>] (do_splice_to+0x78/0x84)
    [<>] (do_splice_to) from [<>] (splice_direct_to_actor+0xc8/0x1cc)
    [<>] (splice_direct_to_actor) from [<>] (do_splice_direct+0xa0/0xb8)
    [<>] (do_splice_direct) from [<>] (do_sendfile+0x1a8/0x30c)
    [<>] (do_sendfile) from [<>] (SyS_sendfile64+0x104/0x10c)
    [<>] (SyS_sendfile64) from [<>] (ret_fast_syscall+0x0/0x38)

Signed-off-by: Vladimir Zapolskiy <vz@mleia.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>