isdn: rename CONFIG_AVMB1_COMPAT to not look like a Kconfig variable

Since CONFIG_AVMB1_COMPAT is not a Kconfig variable, move it out of the
Kconfig namespace.

Signed-off-by: Robert P. J. Day <rpjday@crashcourse.ca>
Acked-by: Karsten Keil <kkeil@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>