drivers/video/backlight/Kconfig: fix typo "MACH_SAM9...EK" three times

Fix three typos (originally) introduced by commit a9a84c37
("atmel_lcdfb: backlight control").

Two of these typos were introduced in v2.6.25.  (The third was
introduced in 915190f7 ("[ARM] 5614/1: at91: atmel_lcdfb: add
at91sam9g10 support to atmel LCD driver")).  Checking these commits
reveals that the default value of 'y' has never been set automatically
in all releases since v2.6.25! Perhaps this line might as well be
dropped.

Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Cc: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>
Acked-by: Jingoo Han <jg1.han@samsung.com>
Cc: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>