tty: ipwireless: fix missing checks for ioremap

ipw->attr_memory and ipw->common_memory are assigned with the
return value of ioremap. ioremap may fail, but no checks
are enforced. The fix inserts the checks to avoid potential
NULL pointer dereferences.

Signed-off-by: Kangjie Lu <kjlu@umn.edu>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>