mmc: litex_mmc: set PROBE_PREFER_ASYNCHRONOUS

mmc host drivers should have enabled the asynchronous probe option, but
it seems like we didn't set it for litex_mmc when introducing litex mmc
support, so let's set it now.

Tested with linux-on-litex-vexriscv on sipeed tang nano 20K fpga.

Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
Acked-by: Gabriel Somlo <gsomlo@gmail.com>
Fixes: 92e09910

 ("mmc: Add driver for LiteX's LiteSDCard interface")
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20230617085319.2139-1-jszhang@kernel.org
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>