ptp: Fix some locking bugs in ptp_read()

In ptp_read there is an unlock missing on an error path, and a double
unlock on another error path.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Acked-by: Richard Cochran <richard.cochran@omicron.at>
Signed-off-by: John Stultz <john.stultz@linaro.org>