bna: remove obsolete use of EXTRA_CFLAGS

EXTRA_CFLAGS should be used on the command line only.

Since EXTRA_CFLAGS here add only a non-existant path to compiler
include paths (by -I), remove EXTRA_CFLAGS completely.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Signed-off-by: David S. Miller <davem@davemloft.net>