qed: Change locking scheme for VF channel

Each VF employees a lock that's supposed to serialize its usage of the
HW channel for communication with its PF, but the critical section is
ill-defined:

  - VFs currently release the lock whenever the PF response arrives,
    prior to actually processing the reply buffer [which was also supposed
    to have been protected by same lock].

  - The lock would be released on first response, ignoring the possibilty
    the sw flow isn't over [as might be the case of the acquisition flow].
    As a result, the flow would run unprotected and would cause a double
    mutex release [as the additional message completion would release it
    while its actually already free].

Change the flow to have a dedicated function to be called at end of each
flow and release the lock.

Signed-off-by: Yuval Mintz <Yuval.Mintz@qlogic.com>
Signed-off-by: David S. Miller <davem@davemloft.net>