mtd: lpc32xx_slc: fix warnings caused by enabling unprepared clock

mtd: lpc32xx_slc: fix warnings caused by enabling unprepared clock

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

    WARNING: CPU: 0 PID: 1 at drivers/clk/clk.c:727 clk_core_enable+0x2c/0xa4()
    Modules linked in:
    CPU: 0 PID: 1 Comm: swapper Not tainted 4.3.0-rc2+ #201
    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_nand_probe+0x290/0x568)
    [<>] (lpc32xx_nand_probe) from [<>] (platform_drv_probe+0x50/0xa0)
    [<>] (platform_drv_probe) from [<>] (driver_probe_device+0x18c/0x408)
    [<>] (driver_probe_device) from [<>] (__driver_attach+0x70/0x94)
    [<>] (__driver_attach) from [<>] (bus_for_each_dev+0x74/0x98)
    [<>] (bus_for_each_dev) from [<>] (driver_attach+0x20/0x28)
    [<>] (driver_attach) from [<>] (bus_add_driver+0x11c/0x248)
    [<>] (bus_add_driver) from [<>] (driver_register+0xa4/0xe8)
    [<>] (driver_register) from [<>] (__platform_driver_register+0x50/0x64)
    [<>] (__platform_driver_register) from [<>] (lpc32xx_nand_driver_init+0x18/0x20)
    [<>] (lpc32xx_nand_driver_init) from [<>] (do_one_initcall+0x11c/0x1dc)
    [<>] (do_one_initcall) from [<>] (kernel_init_freeable+0x10c/0x1d4)
    [<>] (kernel_init_freeable) from [<>] (kernel_init+0x10/0xec)
    [<>] (kernel_init) from [<>] (ret_from_fork+0x14/0x24)

Signed-off-by: Vladimir Zapolskiy <vz@mleia.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>