mtd: spi-nor: Use the spi_mem_xx() API

mtd: spi-nor: Use the spi_mem_xx() API

The spi_mem_xxx() API has been introduced to replace the
spi_flash_read() one. Make use of it so we can get rid of
spi_flash_read().

Note that using spi_mem_xx() also simplifies the code because this API
takes care of using the regular spi_sync() interface when the optimized
->mem_ops interface is not implemented by the controller.

Signed-off-by: Boris Brezillon <[email protected]>
Reviewed-by: Frieder Schrempf <[email protected]>
Tested-by: Frieder Schrempf <[email protected]>
Signed-off-by: Mark Brown <[email protected]>