staging: vt6655: device_intr check for vif on while loop

vif should never be or go null while in loop.

Fixes race condition where interrupts are late and when
interface is not present.

Signed-off-by: Malcolm Priestley <tvboxspy@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>