ARM: OMAP: Fix warning in pm.c

ARM: OMAP2: Fix warning in pm.c:

arch/arm/mach-omap2/pm.c: In function 'omap2_pm_init':
arch/arm/mach-omap2/pm.c:854: warning: ignoring return value
of 'subsys_create_file', declared with attribute
warn_unused_result

Signed-off-by: Dirk Behme <dirk.behme_at_gmail.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>