gpiolib: extend the critical sections of lookup tables

There are two places in the code where we retrieve a lookup table using
gpiod_find_lookup_table() (which protects the table list with the lookup
table lock) and then use it after the lock is released.

We need to keep the lookup table mutex locked the entire time we're using
the tables. Remove the locking from gpiod_find_lookup_table() and use
guards to protect the code actually using the table objects.

Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>