NFC: fix device-allocation error return

A recent change fixing NFC device allocation itself introduced an
error-handling bug by returning an error pointer in case device-id
allocation failed. This is clearly broken as the callers still expected
NULL to be returned on errors as detected by Dan's static checker.

Fix this up by returning NULL in the event that we've run out of memory
when allocating a new device id.

Note that the offending commit is marked for stable (3.8) so this fix
needs to be backported along with it.

Fixes: 20777bc5 ("NFC: fix broken device allocation")
Cc: stable <stable@vger.kernel.org>	# 3.8
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>