soc: qcom: ipa: move interrupt work structure into IPA structure

Get rid of the global ipa_interrupt_work structure and instead embed
the work structure needed in the IPA context.  With the work
structure embedded, we are able to derive an IPA pointer from the
pointer provided to ipa_interrupt_work_func() and avoid using the
global.

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