doc: mmc: drop 0x prefixes in read/write examples

The patch drops 0x prefixes because all numbers are interpreted as HEX
by default.

Also, it fixes the mismatch between input arguments and output at 'mmc
write' example. Now it's 256 (0x100) blocks.

Signed-off-by: Alexander Shirokov <shirokovalexs@gmail.com>
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>