mtd: spi-nor: Move the WE and wait calls inside Write SR methods

Avoid duplicating code by moving the calls to spi_nor_write_enable() and
spi_nor_wait_till_ready() inside the Write Status Register methods.

Move spi_nor_write_sr() to avoid forward declaration of
spi_nor_wait_till_ready().

Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>