sf: Fix to compute proper sector_size

Upto now flash sector_size is assigned from params which isn't
necessarily a sector size from vendor, so based on the SECT_*
flags from flash_params the erase_size will compute and it will
become the sector_size finally.

Bug report (from Bin Meng):
=> sf probe
SF: Detected SST25VF016B with page size 256 Bytes, erase size 4 KiB,
total 2 MiB, mapped at ffe00000

=> sf erase 0 +100
SF: 65536 bytes @ 0x0 Erased: OK

Signed-off-by: Jagannadha Sutradharudu Teki <jagannadh.teki@gmail.com>
Reported-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>