bnxt_en: implement callbacks for devlink selftests

Add callbacks
=============
.selftest_check: returns true for flash selftest.
.selftest_run: runs a flash selftest.

Also, refactor NVM APIs so that they can be
used with devlink and ethtool both.

Signed-off-by: Vikas Gupta <vikas.gupta@broadcom.com>
Reviewed-by: Andy Gospodarek <gospo@broadcom.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>