mtd: spi-nor: Updated write register for spansion flashes

mtd: spi-nor: Updated write register for spansion flashes

Since the spansion flash parts require status register and configuration register
to be written at the same time,changed write_sr_modify_protection to use write_sr_cr
instead of write_sr.
Changed spansion_quad_enable to retain the old status register values in order to
avoid unsetting of the previously enabled flash protection bits.

Signed-off-by: Anurag Kumar Vulisha <anuragku@xilinx.com>
Reviewed-by: Punnaiah Choudary Kalluri <punnaia@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>