gpio: tpic2810: Make sure cached buffer has consistent status with h/w status

i2c_smbus_write_byte_data() can fail. To ensure the
cached buffer has consistent status with h/w status, don't
update the cached gpio->buffer if write fails.

Also refactor the code a bit by adding a tpic2810_set_mask_bits()
helper and use it to simplify the code.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Reviewed-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>