leds: blinkm: Remove work queue

Now the core implements the work queue, remove it from the driver,
and switch to using brightness_set_blocking op.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Cc: Jan-Simon Moeller <dl9pf@gmx.de>
Signed-off-by: Jacek Anaszewski <j.anaszewski@samsung.com>