dm: fdt: Correct handling of aliases with embedded digits

Since we scan from left to right looking for the first digit, "i2c0" returns
2 instead of 0 for the alias number. Adjust the code to scan from right to
left instead.

Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Heiko Schocher <hs@denx.de>
Reviewed-by: Tom Rini <trini@ti.com>