Intermittent Kernel crash has been observed on probe in
bcm_qspi_mspi_l2_isr() handler when the MSPI spifie interrupt bit
has not been cleared before registering for interrupts.
Fix the driver to move SoC specific custom interrupt handling code
before we register IRQ in probe. Also clear MSPI interrupt status
resgiter prior to registering IRQ handlers.

Fixes: cc20a386 ("spi: iproc-qspi: Add Broadcom iProc SoCs support")
Signed-off-by: Kamal Dasu <kdasu@broadcom.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Link: https://lore.kernel.org/r/20211008203603.40915-3-kdasu.kdev@gmail.comSigned-off-by: Mark Brown <broonie@kernel.org>