[ARM] 3897/1: corgi_bl fix module compiling

Fix module compiling:
WARNING: drivers/video/backlight/corgi_bl.o - Section mismatch:
reference to .init.text: from .data between '$d' (at offset 0x0) and
'bl_mutex'

Cc: Richard Purdie <rpurdie@rpsys.net>

Signed-off-by: Anton Vorontsov <cbou@mail.ru>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>