cramfs: fix bug for wrong filename comparison

"cramfsload uImage_1" succeeds even though the actual file is named
"uImage".

Fix file name comparison when one name is the prefix of the other.

Signed-off-by: Holger Brunck <holger.brunck@keymile.com>
cc: Wolfgang Denk <wd@denx.de>
cc: Albert ARIBAUD <albert.u.boot@aribaud.net>