soc: qcom: ipa: kill ipa_irq_init()

All ipa_irq_init() does is look up the IPA IRQ number.  That can be
done at any time, including at the beginning of ipa_interrupt_init()
where that value is first used.  Move the contents of ipa_irq_init()
into ipa_interrupt_init(), and do the same with ipa_irq_exit().

Signed-off-by: Alex Elder <elder@linaro.org>