bnxt_en: Refactor code that determines RFS capability.

Add function bnxt_rfs_supported() that determines if the chip supports
RFS.  Refactor the existing function bnxt_rfs_capable() that determines
if run-time conditions support RFS.

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