sf: Correct bank select incase of dual stacked

Correct the bank selection issue incase of Dual stacked mode.
This fix corrects the wrong bank selection if banks are accessed
as below.
1. Access the bank2 in upper flash.
2. Access the bank1 in lower flash.
3. Now access the bank1 in upper flash.
But here in the step3, the present code was accessing the bank2 in
upper flash not bank1. This was because the code thinks the bank1
was already selected as part of step2 but it was not taking care of
upper or lower flash.

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