spi: mt65xx: Add and use pointer to struct device in mtk_spi_probe()

In preparation for switching to dev_err_probe() in this function, add
a pointer to struct device and replace all occurrences of '&pdev->dev'
to using this 'dev' pointer.
This is done for one-line fitting of the dev_err_probe() calls.

Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://lore.kernel.org/r/20220407114428.167091-4-angelogioacchino.delregno@collabora.com


Signed-off-by: Mark Brown <broonie@kernel.org>