drivers: sdhc: esp32: fix sd card init hang on command write

The sdmmc_ll_set_command helper writes the command register
through memcpy, which the compiler expands into four byte
stores. The peripheral bus only accepts 32-bit accesses, so
the command never reached the card interface unit and the
driver spun forever waiting for it to be taken.

Restore the single word store to the command register. This
is a regression from f4c7092ad64.

Signed-off-by: Sylvio Alves <sylvio.alves@espressif.com>