ARM: OMAP: Fix warnings in plat-omap

Fix warnings

arch/arm/plat-omap/dmtimer.c: In function
'omap_dm_timer_modify_idlect_mask':
arch/arm/plat-omap/dmtimer.c:317: warning: no return
statement in function returning non-void

arch/arm/plat-omap/mailbox.c: In function 'omap_mbox_init':
arch/arm/plat-omap/mailbox.c:231: warning: ignoring return
value of 'class_device_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>