mmc: seperate block number into small parts for multi-write cmd

Constraint the mmc framework to only send no more than 65535
blocks in one go during the multi-write command. This constraint
comes due to the limitation of 16bit width block counter register
at some hardware.

Signed-off-by: Lei Wen <leiwen@marvell.com>
Cc: Reinhard Meyer <u-boot@emk-elektronik.de>