arcnet: Neaten BUGMSG macro defines

These macros are actually printk and pr_cont uses with a flag.

Add a new BUGLVL_TEST macro which is just the "should use" test
and not an odd "if (<foo>)" macro to simplify uses in a new patch.

Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: Michael Grzeschik <m.grzeschik@pengutronix.de>