net/ixgbe: fix mailbox interrupt handler

net/ixgbe: fix mailbox interrupt handler

Mailbox interrupt handler only takes care of PF reset notification, for
other message ixgbe_read_mbx should not be called since it gets chance
to break the foreground VF to PF communication.
This can be simply repeated by 'testpmd>rx_vlan rm all 0'.

Fixes: 77234603fba0 ("net/ixgbe: support VF mailbox interrupt for link up/down")
Cc: stable@dpdk.org
Signed-off-by: Qi Zhang <qi.z.zhang@intel.com>
Acked-by: Wei Dai <wei.dai@intel.com>