mmc: loongson2: prevent integer overflow in ret variable

mmc: loongson2: prevent integer overflow in ret variable

In loongson2_mmc_dll_mode_init(), `ret` variable is declared
as u32 but it is expected to hold an int value.

Fixes: d0f8e961deae ("mmc: loongson2: Add Loongson-2K2000 SD/SDIO/eMMC controller driver")
Reported-by: https://scan7.scan.coverity.com/#/project-view/53936/11354?selectedIssue=1644958
Signed-off-by: Sergio Perez Gonzalez <sperezglz@gmail.com>
Reviewed-by: Binbin Zhou <zhoubinbin@loongson.cn>
Link: https://lore.kernel.org/r/20250707185545.46275-1-sperezglz@gmail.com
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>