moxa: prevent opening unavailable ports

In moxa.c there are 32 minor numbers reserved for each device. The
number of ports actually available per device is stored in
moxa_board_conf->numPorts. This number is not considered in moxa_open().
Opening a port that is not available results in a kernel oops.
This patch adds a test to moxa_open() that prevents opening unavailable
ports.

Signed-off-by: Dirk Eibach <eibach@gdsys.de>
Signed-off-by: Jiri Slaby <jirislaby@gmail.com>
Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>