ARM: cleanup: soc_device_register() error checking

soc_device_register() never returns NULL, it only ever returns an
error pointer or a valid pointer.  Use the right function (IS_ERR())
to check this.

soc_device_to_device() only ever returns &soc_dev->dev, and so can
never return an error or NULL if the pointer passed into it was
valid, so there's no point checking its return.

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