bnxt_en: Refactor logic to re-enable SRIOV after firmware reset detected.

Put the current logic in bnxt_open() to re-enable SRIOV after detecting
firmware reset into a new function bnxt_reenable_sriov().  This call
needs to be invoked in the firmware reset path also in the next patch.

Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>