staging: vchiq_core: Fix missing semaphore release in error case

The bail out branch in case of a invalid tx_pos missed a semaphore
release. Dan Carpenter found this with a static checker.

Fixes: d1eab9de ("staging: vchiq_core: Bail out in case of invalid tx_pos")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>