[PATCH] USB: usbserial: Adds missing checks and bug fix.

Checks if 'port' is NULL before using it in all tty operations, this
can avoid NULL pointer dereferences.

Signed-off-by: Luiz Capitulino <lcapitulino@mandriva.com.br>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>