The read check is processing a buffer 10 blocks long. The check then
reads data from the SD card 10 times to then report the speed of
"Read 10 blocks at X bytes/second". The problem is that it actually
reads 10*10 blocks.

With this patch, read_blocks is used correctly to populate a continuous
buffer.

Signed-off-by: Petr Horacek <[email protected]>

`write_block` needs to be called for every 512 bytes long block that is
to be written to the SD card. Each of these calls requires the overhead
of negotiating the write command.

With this patch, `write_blocks` is introduced, mirroring the behavior
of `read_block` and `read_blocks`. This new method helped me to improve
write speed of the original 9492 kB/s to 66252 kB/s.

Signed-off-by: Petr Horacek <[email protected]>