crc: Fix code style with crc functions

Some of these have a space before the bracket. Drop it to fix the style.
Add some missing function comments while here.

Note that u32 and u8 cannot be used here since crc.h is included on the
host side.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>