mmc: block: Change MMC_IOC_MAX_BYTES

It is used for limitation of buffer size during IOCTL such as FFU.
However, eMMC FW size is bigger than (512L*256).
(For instance, currently, Samsung eMMC FW size is over 300KB.)
So, it needs to increase to execute FFU.

Signed-off-by: Jeonghan Kim <jh4u.kim@samsung.com>
Acked-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>