bnxt_en: Convert to use netif_level() helpers.

Use the various netif_level() helpers to simplify the C code.  This was
suggested by Joe Perches.

Cc: Joe Perches <joe@perches.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Link: https://lore.kernel.org/r/1611642024-3166-1-git-send-email-michael.chan@broadcom.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>