mtd: spi-nor: factor out write_enable() for erase commands

write_enable() was being duplicated to both m25p80.c and fsl-quadspi.c.
But this should be handled within the spi-nor abstraction layer.

At the same time, let's add write_disable() after erasing, so we don't
leave the flash in a write-enabled state afterward.

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Acked-by: Huang Shijie <shijie.huang@intel.com>