W1: w1_int.c use first available master number

Follow the example of other devices (like the joystick device).  Pick the
first available id for each detected device.  Currently for USB devices,
suspending and resuming would cause the number to increment.

Signed-off-by: David Fries <david@fries.net>
Signed-off-by: Evgeniy Polyakov <johnpol@2ka.mipt.ru>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>