drivers/misc/ab8500-pwm.c: fix modalias

Since 43cc71ee ("platform: prefix MODALIAS with "platform:""), the
platform modalias is prefixed with "platform:".

This patch changes the MODULE_ALIAS to "platform:ab8500-pwm".

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Acked-by: Arun Murthy <arun.murthy@stericsson.com>
Cc: Linus Walleij <linus.walleij@stericsson.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>