hw/m25p80.c: add WRSR(0x01) support

Atmel, SST and Intel/Numonyx serial flash tend to power up
with the software protection bits set.
And thus the new m25p80.c in linux kernel would always tries
to use WREN(0x06) + WRSR(0x01) to turn-off the protection.

The WEL(0x02) of status register is supposed to be cleared after
WRSR(0x01). There are also some drivers (i.e mine for RTOSes)
would check the WEL(0x02) in status register to make sure the
protection is correctly turned off.

Signed-off-by: Kuo-Jung Su <dantesu@faraday-tech.com>
Cc: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Cc: Peter Maydell <peter.maydell@linaro.org>
Cc: Edgar E. Iglesias <edgar.iglesias@gmail.com>
Reviewed-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Signed-off-by: Edgar E. Iglesias <edgar.iglesias@gmail.com>
