mtd: spi-nor: Use write_sr for write status

Use existing write_sr() call instead of decoding and
calling nor->write_reg separately.

Signed-off-by: Jagan Teki <jteki@openedev.com>
Cc: David Woodhouse <dwmw2@infradead.org>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>