cmd_fuse: return CMD_RET_FAILURE on error

Fuse drivers, like the mxs_ocotp.c, may return negative error codes but
the commands are only allowed to return CMD_RET_* enum values to the
shell, otherwise the following error appears:

	"exit not allowed from main input shell."

Signed-off-by: Hector Palacios <hector.palacios@digi.com>
Reviewed-by: Benoît Thébaudeau <benoit.thebaudeau.dev@gmail.com>