spi: bcm-qspi: Use platform_get_irq_byname_optional() to avoid error message

Use platform_get_irq_byname_optional() instead of platform_get_irq_byname()
to avoid below error message during probe:

[3.265115] bcm_iproc 68c70200.spi: IRQ spi_lr_fullness_reached not found
[3.272121] bcm_iproc 68c70200.spi: IRQ spi_lr_session_aborted not found
[3.284965] bcm_iproc 68c70200.spi: IRQ spi_lr_impatient not found
[3.291344] bcm_iproc 68c70200.spi: IRQ spi_lr_session_done not found
[3.297992] bcm_iproc 68c70200.spi: IRQ mspi_done not found
[3.303742] bcm_iproc 68c70200.spi: IRQ mspi_halted not found

Signed-off-by: Rayagonda Kokatanur <rayagonda.kokatanur@broadcom.com>
Link: https://lore.kernel.org/r/20200107040912.16426-1-rayagonda.kokatanur@broadcom.com


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