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>
diff --git a/drivers/mtd/spi/sf_internal.h b/drivers/mtd/spi/sf_internal.h
index bd834dc..4158e13 100644
--- a/drivers/mtd/spi/sf_internal.h
+++ b/drivers/mtd/spi/sf_internal.h
@@ -119,7 +119,8 @@
  * @name:		Device name ([MANUFLETTER][DEVTYPE][DENSITY][EXTRAINFO])
  * @jedec:		Device jedec ID (0x[1byte_manuf_id][2byte_dev_id])
  * @ext_jedec:		Device ext_jedec ID
- * @sector_size:	Sector size of this device
+ * @sector_size:	Isn't necessarily a sector size from vendor,
+ *			the size listed here is what works with CMD_ERASE_64K
  * @nr_sectors:	No.of sectors on this device
  * @e_rd_cmd:		Enum list for read commands
  * @flags:		Important param, for flash specific behaviour