bnxt_en: Add support to call FW to update a VNIC

Add the function bnxt_hwrm_vnic_update() to call FW to update
a VNIC.  This call can be used when disabling and enabling a
receive ring within a VNIC.  The mru which is the maximum receive
size of packets received by the VNIC can be updated.

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>