ipwireless: fix potential tty == NULL dereference

The Coverity checker spotted the following inconsequent NULL checking in
drivers/char/pcmcia/ipwireless/network.c:ipwireless_network_packet_received()

if (tty && channel_idx == IPW_CHANNEL_RAS
		&& (network->ras_control_lines &
			IPW_CONTROL_LINE_DCD) != 0
		&& ipwireless_tty_is_modem(tty)) {
...
	else
		ipwireless_tty_received(tty, data, length);

Cc: Adrian Bunk <bunk@kernel.org>
Signed-off-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>
Cc: "John W. Linville" <linville@tuxdriver.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>