The tty_find_polling_driver() routine did not correctly check the base
part of the tty name.  This can lead to kgdboc selecting an incorrect
driver, as well as accepting a completely invalid tty such as "echo
ffff0 > /sys/module/kgdboc/parameters/kgdboc".
Signed-off-by: Jason Wessel <jason.wessel@windriver.com>