gpio: pca953x: Rewrite ->get_multiple() function

The commit 96d7c7b3

 ("gpio: gpio-pca953x, Add get_multiple function")
basically did everything wrong from style and code reuse perspective, i.e.
- it didn't utilize existing PCA953x internal helpers
- it didn't utilize bitmap API
- it misses the point that ilog2(), besides that BANK_SFT is useless,
  can be used in macros
- it has indentation issues.

Rewrite the function completely.

Cc: Paul Thomas <pthomas8589@gmail.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>