spi: mtk_spim: clear IRQ enable bits

spi: mtk_spim: clear IRQ enable bits

In u-boot we don't use IRQ. Instead, we poll busy bit in SPI_STATUS.

However these IRQ enable bits may be set in previous boot stage (BootROM).

If we leave these bits not cleared, although u-boot has disabled IRQ and
nothing will happen, the linux kernel may encounter panic during
initializing the spim driver due to IRQ event happens before IRQ handler
is properly setup.

This patch clear IRQ bits to prevent this from happening.

Signed-off-by: SkyLake.Huang <skylake.huang@mediatek.com>
Signed-off-by: Weijie Gao <weijie.gao@mediatek.com>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>