bnxt_en: Refactor bnxt_sriov_enable().

Refactor the hardware/firmware configuration portion in
bnxt_sriov_enable() into a new function bnxt_cfg_hw_sriov().  This
new function can be called after a firmware reset to reconfigure the
VFs previously enabled.

v2: straight refactor of the code.  Reordering done in the next patch.

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