mtd: cfi_cmdset_0002: make sector erase command variable

Some old SST chips use 0x50 as sector erase command, instead
of 0x30. Make this value variable to handle such chips.

Signed-off-by: Guillaume LECERF <glecerf@gmail.com>
Acked-by: Wolfram Sang <w.sang@pengutronix.de>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>