net/mlxfw: Properly handle dependancy with non-loadable mlx5

If mlx5 is set to be built-in and mlxfw as a module, we
get a link error:

drivers/built-in.o: In function `mlx5_firmware_flash':
(.text+0x5aed72): undefined reference to `mlxfw_firmware_flash'

Since we don't want to mandate selecting mlxfw for mlx5 users, we
use the IS_REACHABLE macro to make sure that a stub is exposed
to the caller.

Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Reported-by: Jakub Kicinski <kubakici@wp.pl>
Reported-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>