mmc: stm32_sdmmc2: increase polling status register delay

MMC commands like MMC_CMD_ALL_SEND_CID or MMC_CMD_SEND_CSD can reach
500 us. This patch increases the polling status register delay to avoid
a timeout on a command.

Signed-off-by: Christophe Kerello <christophe.kerello@st.com>
Signed-off-by: Patrice Chotard <patrice.chotard@st.com>