i2c-algo-pcf: Handle timeout correctly

With a postfix decrement these timeouts reach -1 rather than 0, but after the
loop it is tested whether they have become 0.

As pointed out by Jean Delvare, the msg_num should be tested before the timeout.
With the current order, you could exit with a timeout error while all the
messages were successfully transferred.

Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
Signed-off-by: Jean Delvare <khali@linux-fr.org>
Acked-by: Eric Brower <ebrower@gmail.com>