[media] staging: go7007: fix mismatch in mutex lock-unlock in [read|write]_reg_fp

If go7007_usb_vendor_request() fails in write_reg_fp()
or in read_reg_fp(), the usb->i2c_lock mutex left locked.

The patch moves mutex_unlock(&usb->i2c_lock) before check
for go7007_usb_vendor_request() returned value.

Found by Linux Driver Verification project (linuxtesting.org).

Signed-off-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>