x86: gpio: Tidy up gpio_ich6_get_base() and callers

x86: gpio: Tidy up gpio_ich6_get_base() and callers

This function can return an error. Correct the detection of this error so
that it works even with large 32-bit addresses.

The return value is set up for returning an I/O address but the function is
also used to return a memory-mapped address. Adjust the return code to make
this work.

Also add a bit more debugging.

Signed-off-by: Simon Glass <sjg@chromium.org>