cpufreq: OMAP: Check IS_ERR() instead of NULL for omap_device_get_by_hwmod_name

omap_device_get_by_hwmod_name() returns ERR_PTR on error.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Acked-by: Kevin Hilman <khilman@ti.com>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>