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

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

Fix three typos (originally) introduced by a9a84c37d ("atmel_lcdfb:
backlight control").

Two of these typos were introduced in v2.6.25.  (The third was introduced
in 915190f7d4f08 ("[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>