lib: strto: fix metric suffix parsing in strtoul[l]

While 1kB or 1kiB will be parsed correctly, 1k will return the right
amount, but the metric suffix will not be escaped once the char
pointer updated. Fix this situation by simplifying the move of the
endp pointer.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Stefan Roese <sr@denx.de>