mmc: rockchip_sdhci: Do not test unsigned for being less than 0

mmc: rockchip_sdhci: Do not test unsigned for being less than 0

In rockchip_sdhci_execute_tuning the variable tuning_loop_counter is
tested for being less than 0. Ensure that it is a signed type by
declaring it as s8 instead of char.

This issue was found by Smatch.

Signed-off-by: Andrew Goodbody <andrew.goodbody@linaro.org>
Reviewed-by: Peng Fan <peng.fan@nxp.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>