bnxt_en: Check the FW's VNIC flush capability

Check the HWRM_VNIC_QCAPS FW response for the receive engine flush
capability.  This capability indicates that we can reliably support
RX ring restart when calling HWRM_VNIC_UPDATE with MRU set to 0.

Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David Wei <dw@davidwei.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>