w1: keep balance of mutex locks and refcnts

w1_therm_eeprom() and w1_DS18B20_precision() decrement THERM_REFCNT
on error paths, while they did not increment it yet.

read_therm() unlocks bus mutex on some error paths,
while it is not acquired.

The patch makes sure all the functions keep the balance in usage of
the mutex and the THERM_REFCNT.

Found by Linux Driver Verification project (linuxtesting.org).

Signed-off-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>