Char: mxser, fix TIOCMIWAIT

There was schedule() missing in the TIOCMIWAIT ioctl. Solve it by moving
the code to the wait_event_interruptible.

Signed-off-by: Jiri Slaby <jirislaby@gmail.com>
Cc: Jan Yenya Kasprzak <kas@fi.muni.cz>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>