hw/sd: Rename read/write_data() as read/write_byte()

The read/write_data() methods write do a single byte access
on the data line of a SD card. Rename them as read/write_byte().
Add some documentation (not in "hw/sd/sdcard_legacy.h" which we
are going to remove soon).

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20200814092346.21825-2-f4bug@amsat.org>