backlight: convert backlight spi drivers to module_spi_driver

Factor out some boilerplate code for spi driver registration into
module_spi_driver.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Acked-by: Jingoo Han <jg1.han@samsung.com>
Cc: Eric Miao <eric.y.miao@gmail.com>
Acked-by: Alberto Panizzo <alberto@amarulasolutions.com>
Cc: Donghwa Lee <dh09.lee@samsung.com>
Acked-by: Marek Vasut <marek.vasut@gmail.com>
Cc: Haavard Skinnemoen <hskinnemoen@atmel.com>
Cc: InKi Dae <inki.dae@samsung.com>
Cc: Dmitry Baryshkov <dbaryshkov@gmail.com>
Cc: Ben Dooks <ben-linux@fluff.org>
Cc: Richard Purdie <rpurdie@rpsys.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>