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

Get rid of the global tx_suspend_work delayed work structure and
instead embed such a structure in the IPA context.  With the work
structure embedded, we are able to derive an IPA pointer from the
pointer provided to enable_tx_suspend_work_func() and avoid using
the global.

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