uboot-imx

mmc: Print send_cmd response only when return value is zero

send_cmd response is valid only when no error happened. If an error
occured, let mmc_send_cmd() print the return value to aid debugging.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Tom Rini <trini@konsulko.com>