mtd: spi-nor: Stop compare with negative in Reg Ops methods

spi_mem_exec_op()
nor->controller_ops->write_reg()
nor->controller_ops->read_reg()
spi_nor_wait_till_ready()
Return 0 on success, -errno otherwise.

Stop compare with negative and compare with zero in all the register
operations methods.

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