mtd: m25p80: disable SST software protection bits by default

The SST SPI flashes is like Atmel SPI flashes in that the software
protection bits are set by default at power up, so clear them at init
time.

Signed-off-by: Graf Yang <graf.yang@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>