sf: Fix the remain_len calculation in Dual stacked

The bank_sel should be increased by total banks in first
chip, if accessing the second chip in Dual stacked mode.
This fixes the wrong remain_len calculation if two 16MB
chips connected in Dual stacked and accessing the second
chip.

Signed-off-by: Siva Durga Prasad Paladugu <sivadur@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>