bnxt_en: prepare to refactor ethtool reset types

Extract bnxt_hwrm_firmware_reset() for performing firmware reset
operations. This new helper function will be used in a subsequent
patch to separate unrelated reset types out of bnxt_firmware_reset().

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