char: 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.

[akpm@linux-foundation.org: avoid multiple returns]
Signed-off-by: Dirk Eibach <eibach@gdsys.de>
Signed-off-by: Jiri Slaby <jirislaby@gmail.com>
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>