spi: ich: Various small tidy-ups

Use debug() instead of printf() to reduce code size and change a bool
return value to the use the 'bool' type. Also drop the global data
declaration since it not actually used. Finally, set the log category.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>