msm: ipa3: do not clear irq before register

Avoid clearing pending interrupts on IPA driver before registering.
This may lead to missing an interrupt fired before IPA was
registered to it.

Change-Id: I69a0374eb55d4fcc79d87a1acb7aa73ae6807439
CRs-Fixed: 965412
Acked-by: Ady Abraham <adya@qti.qualcomm.com>
Signed-off-by: Skylar Chang <chiaweic@codeaurora.org>