[media] cxusb: unlock on error in cxusb_i2c_xfer()

We recently introduced some new error paths which are missing their
unlocks.
Fixes: 64f7ef8afbf8 ('[media] cxusb: Don't use dynamic static allocation')

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>