spi: atmel-quadspi: void return type for atmel_qspi_init()

commit 2d30ac5e ("mtd: spi-nor: atmel-quadspi: Use spi-mem interface for atmel-quadspi driver")
removed the error path from atmel_qspi_init(), but not changed the
function's return type. Set void return type for atmel_qspi_init().

Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Signed-off-by: Mark Brown <broonie@kernel.org>