spi: spi-nxp-fspi: fix fspi panic by unexpected interrupts

spi: spi-nxp-fspi: fix fspi panic by unexpected interrupts

Given the case that bootloader(such as UEFI)'s FSPI driver might not
handle all interrupts before loading kernel, those legacy interrupts
would assert immidiately once kernel's FSPI driver enable them. Further,
if it was FSPI_INTR_IPCMDDONE, the irq handler nxp_fspi_irq_handler()
would call complete(&f->c) to notify others. However, f->c might not be
initialized yet at that time, then cause kernel panic.

Of cause, we should fix this issue within bootloader. But it would be
better to have this pacth to make dirver more robust (by clearing all
interrupt status bits before enabling interrupts).

Suggested-by: Han Xu <han.xu@nxp.com>
Signed-off-by: Ran Wang <ran.wang_1@nxp.com>
Link: https://lore.kernel.org/r/20201123025715.14635-1-ran.wang_1@nxp.com
Signed-off-by: Mark Brown <broonie@kernel.org>