MAINTAINERS: correct entries for ISDN/mISDN section

Commit 6d979850 ("isdn: move capi drivers to staging") cleaned up the
isdn drivers and split the MAINTAINERS section for ISDN, but missed to add
the terminal slash for the two directories mISDN and hardware. Hence, all
files in those directories were not part of the new ISDN/mISDN SUBSYSTEM,
but were considered to be part of "THE REST".

Rectify the situation, and while at it, also complete the section with two
further build files that belong to that subsystem.

This was identified with a small script that finds all files belonging to
"THE REST" according to the current MAINTAINERS file, and I investigated
upon its output.

Fixes: 6d979850 ("isdn: move capi drivers to staging")
Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>