USB: serial: option: add interface-number sanity check to flag handling

Add an interface-number sanity check before testing the device flags to
avoid relying on undefined behaviour when left shifting in case a device
uses an interface number greater than or equal to BITS_PER_LONG (i.e. 64
or 32).

Reported-by:  <syzbot+8881b478dad0a7971f79@syzkaller.appspotmail.com>
Fixes: c3a65808

 ("USB: serial: option: reimplement interface masking")
Cc: stable@vger.kernel.org
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>