W1: w1_therm: release the bus during conversion on externally powered devices

w1_therm devices can either be bus powered or externally powered.

When device is bus powered during temperature conversion the bus
have to be left high to provide necessary power. Some masters also allow
strong power-up to be enabled in this case.
Naturally, no communication over bus can occur during that time.

However, if device has external power then there is no such restriction,
and host can talk to other devices during temperature conversion.

There is command which allows us to check how device is powered,
this patch uses it to release the bus on externally w1_therm powered devices
during temperature conversion.

Also, this changes uninterruptible sleeps there into interruptible ones to
avoid long uninterruptible sleep if w1 subsystem happens to grab bus for
scan during w1_therm_read().

Signed-off-by: Maciej Szmigiero <mhej@o2.pl>
Acked-by: Evgeniy Polyakov <zbr@ioremap.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>