qed: Unlock on error in qed_vf_pf_acquire()

My static checker complains that we're holding a mutex on this error
path.  Let's goto exit instead of returning directly.

Fixes: b0bccb69

 ("qed: Change locking scheme for VF channel")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Yuval Mintz <Yuval.Mintz@cavium.com>
Signed-off-by: David S. Miller <davem@davemloft.net>