extcon: qcom-spmi: Switch to platform_get_irq_byname_optional

Valid configurations for the extcon interrupt declarations are

- usb_id
- usb_vbus
- (usb_id | usb_vbus)

In the case of a standalone usb_id or usb_vbus failure to find one of the
interrupts shouldn't generate a warning message.

Switch to using platform_get_irq_byname_optional() in order to facilitate
this behaviour.

Suggested-by: Marijn Suijten <marijn.suijten@somainline.org>
Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>