eth: bnxt: add missing netdev lock management to bnxt_dl_reload_up

eth: bnxt: add missing netdev lock management to bnxt_dl_reload_up

bnxt_dl_reload_up is completely missing instance lock management
which can result in `devlink dev reload` leaving with instance
lock held. Add the missing calls.

Also add netdev_assert_locked to make it clear that the up() method
is running with the instance lock grabbed.

v2:
- add net/netdev_lock.h include to bnxt_devlink.c for netdev_assert_locked

Fixes: 004b5008016a ("eth: bnxt: remove most dependencies on RTNL")
Signed-off-by: Stanislav Fomichev <sdf@fomichev.me>
Link: https://patch.msgid.link/20250309215851.2003708-3-sdf@fomichev.me
Signed-off-by: Jakub Kicinski <kuba@kernel.org>