spi: rspi: Use platform_get_irq_byname_optional() for optional irqs

As platform_get_irq_byname() now prints an error when the interrupt
does not exist, scary warnings may be printed for optional interrupts:

    renesas_spi e6b10000.spi: IRQ rx not found
    renesas_spi e6b10000.spi: IRQ mux not found

Fix this by calling platform_get_irq_byname_optional() instead.
Remove the no longer needed printing of platform_get_irq errors, as the
remaining calls to platform_get_irq() and platform_get_irq_byname() take
care of that.

Fixes: 7723f4c5 ("driver core: platform: Add an error message to platform_get_irq*()")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Stephen Boyd <swboyd@chromium.org>
Link: https://lore.kernel.org/r/20191016143101.28738-1-geert+renesas@glider.be


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