mmc: litex_mmc: ensure `host->irq == 0` if polling

Ensure the flag is explicitly set to 0 if we determine that polling is
needed during driver probe, to cover all possible cases.

Fixes: 92e09910 ("mmc: Add driver for LiteX's LiteSDCard interface")
Signed-off-by: Gabriel Somlo <gsomlo@gmail.com>
Link: https://lore.kernel.org/r/20221107155516.2535912-1-gsomlo@gmail.com


Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>