phy: renesas: rcar-gen3-usb2: Use platform_get_irq_optional() for optional irq

As platform_get_irq() now prints an error when the interrupt does not
exist, a scary warning may be printed for an optional interrupt:

    phy_rcar_gen3_usb2 ee0a0200.usb-phy: IRQ index 0 not found

Fix this by calling platform_get_irq_optional() instead.

Fixes: 7723f4c5 ("driver core: platform: Add an error message to platform_get_irq*()")
Reviewed-by: Stephen Boyd <swboyd@chromium.org>
Reviewed-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Tested-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>