nfc: signedness bug in __nci_request()

wait_for_completion_interruptible_timeout() returns -ERESTARTSYS if
interrupted so completion_rc needs to be signed.  The current code
probably returns -ETIMEDOUT if we hit this situation, but after this
patch is applied it will return -ERESTARTSYS.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>