msm: ipa: use global workqueue for immediate command TX completion

TX completion for data pipe and immediate command pipe were using
the same workqueue (with max_active of 1) so immediate command
completion could be delayed by data completion (which works in
polling mode for long durations). This change uses the global
workqueue for the immediate command completion.

Change-Id: Ie5a4a247ab4ee842f616235caeca5b541b8c31ef
Signed-off-by: Ravi Gummadidala <rgummadi@codeaurora.org>