mmc: sdhci: fix the wrong type of curr

curr should use signed type since it will contain the returned
value which is possible to be a negative value. Using u32 will
make the returned value to be true even there is a negative result.
Change to use int instead of u32

Signed-off-by: Chuanxiao Dong <chuanxiao.dong@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>