media: dvb-usb-v2: af9035: fix missing unlock

Instead of returning an error, goto the mutex unlock at
the end of the function.

Fixes smatch warning:

drivers/media/usb/dvb-usb-v2/af9035.c:467 af9035_i2c_master_xfer() warn: inconsistent returns '&d->i2c_mutex'.
  Locked on  : 326,387
  Unlocked on: 465,467

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Fixes: 7bf744f2

 ("media: dvb-usb-v2: af9035: Fix null-ptr-deref in af9035_i2c_master_xfer")
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>