sd: sdmmc: initialize cmd retries

Sets the retries in cmd struct to their Kconfig value. This fixes
an issue in imx_usdhc.c where retries would be used uninitialized. That
leads to a loop not being entered and subsequently no data being read
from the controller.
Tested on mimxrt1170_evk.

Signed-off-by: Jakob Genßler <jakob.genssler@gin.de>