gpio: 74x164: Driver cleanup

- Removed excess "spi" field from private data.
- Removed excess check for spi_get_drvdata() in remove().
- Removed unneeded message in remove().
- Simplify error path in probe().
- Fixed warnings by scripts/checkfile.pl.

Signed-off-by: Alexander Shiyan <shc_work@mail.ru>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>