xhci: hsic: Offload event handling to softirq context

Move event handling from hard IRQ context to a tasklet.
Also limit the maximum number of events handled to 20 by
tasklet. This helps in reducing xhci irq latency in
handling events per irq.

CRs-Fixed: 704032
Change-Id: I071778efe13e7ac9a148e16266d7dd79d139eb4d
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>