lib: strto: fix incorrect handling of specified base

The strto functions should honor the specified base (if non-zero) rather
than permitting a hex or octal string when the user wanted (for example)
base 10.

This has been fixed somewhere along the way in the upstream linux kernel
src tree, at some point after these was copied in to u-boot.  And also
in a way that duplicates less code.  So port _parse_integer_fixup_radix()
to u-boot.

Signed-off-by: Rob Clark <robdclark@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>