mmc/dw_mmc: Allocate the correct amount of descriptors

This fixes two issues:
 * a descriptor was allocated for every block, while a descriptor can
   take 8 blocks
 * there was an off-by-one error in the descriptor preparation: there
   were two last descriptors, one with length==0

Signed-off-by: Mischa Jonker <mjonker@synopsys.com>
Cc: Alexey Brodkin <abrodkin@synopsys.com>
Cc: Jaehoon Chung <jh80.chung@samsung.com>
Cc: Andy Fleming <afleming@gmail.com>