spi: mediatek: Enable irq when pdata is ready

If the device does not come straight from reset, we might receive an IRQ
before we are ready to handle it.

Fixes:

[    0.832328] Unable to handle kernel read from unreadable memory at virtual address 0000000000000010
[    1.040343] Call trace:
[    1.040347]  mtk_spi_can_dma+0xc/0x40
...
[    1.262265]  start_kernel+0x338/0x42c

Signed-off-by: Ricardo Ribalda <ribalda@chromium.org>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://lore.kernel.org/r/20221128-spi-mt65xx-v1-0-509266830665@chromium.org
Signed-off-by: Mark Brown <broonie@kernel.org>