ARM: cleanup: OMAP hwmod error checking

omap_hwmod_lookup() only returns NULL on error, never an error pointer.
Checking the returned pointer using IS_ERR_OR_NULL() is needless
overhead.  Use a simple !ptr check instead.

OMAP devices (oh->od) always have a valid platform device attached (see
omap_device_alloc()) so there's no point validating the platform device
pointer (we will have already oopsed long before if this is not the
case here.)

Lastly, oh->od is only ever NULL or a valid omap device pointer - 'oh'
comes from the statically declared hwmod tables, and the pointer is
only filled in by omap_device_alloc() at a point where the omap device
pointer must be valid.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>