leds: Rename brightness_set_sync op to brightness_set_blocking

leds: Rename brightness_set_sync op to brightness_set_blocking

The initial purpose of brightness_set_sync op, introduced along with
the LED flash class extension, was to add a means for setting torch LED
brightness as soon as possible, which couldn't have been guaranteed by
brightness_set op. This patch renames the op to brightness_set_blocking,
which describes its purpose in a more generic way. It is beneficial
in view of the prospective changes in the LED core, aiming at removing
the need for using work queues in LED class drivers that can sleep
or use delays while setting brightness.

Signed-off-by: Jacek Anaszewski <j.anaszewski@samsung.com>
Acked-by: Andrew Lunn <andrew@lunn.ch>
Acked-by: Pavel Machek <pavel@ucw.cz>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>