lib: Drop unnecessary check for hex digit

If we see 0x then we can assume this is the start of a hex value. It
does not seem necessary to check for a hex digit after that since it will
happen when parsing the value anyway.

Drop this check to simplify the code and reduce size. Add a few more test
cases for when a 0x prefix is used.

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