mmc: arm_pl180: Limit data transfer to U16_MAX

Currently fetching files bigger that cause a data transfer greater than
U16_MAX fails.

The reason is that the specification defines the datalength register
as a 16 bit wide register, but in u-boot it is used as if it is an
32 bit register. Therefore values greater than U16_MAX cause an
infinite loop inside u-boot. U-boot expects to get more data from
interface/hardware then it will ever get and therefore inifintely waits
for more data that will never come.

Signed-off-by: Maximilian Brune <maximilian.brune@9elements.com>
Cc: Peng Fan <peng.fan@nxp.com>
Cc: Jaehoon Chung <jh80.chung@samsung.com>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>