cxgb4vf: fix mailbox data/control coherency domain race

For the VFs, the Mailbox Data "registers" are actually backed by
T4's "MA" interface rather than PL Registers (as is the case for
the PFs).  Because these are in different coherency domains, the
write to the VF's PL-register-backed Mailbox Control can race in
front of the writes to the MA-backed VF Mailbox Data "registers".
So we need to do a read-back on at least one byte of the VF Mailbox
Data registers before doing the write to the VF Mailbox Control
register.

Signed-off-by: Casey Leedom <leedom@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>