mmc: matsushita-common: Wait for command completion

Make sure to wait for the command to complete altogether, including
the trailing 8 clock cycles. This prevents the driver for accidentally
writing the CMD register too fast before the previous command fully
completed.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Jaehoon Chung <jh80.chung@samsung.com>
Cc: Masahiro Yamada <yamada.masahiro@socionext.com>