mtd: spi-nor: Zap unneeded write_enable from write_reg

The 'write_enable' argument is unused and unneeded, so remove it from
the API.

Signed-off-by: Jagan Teki <jteki@openedev.com>
Cc: David Woodhouse <dwmw2@infradead.org>
Cc: Han Xu <han.xu@freescale.com>
[Brian: fixed for nxp-spifi.c]
Signed-off-by: Brian Norris <computersforpeace@gmail.com>