power: pm8921-charger: init workqueues before requesting interrupts

There is a possible race condition where an uninitialized work is
scheduled when an interrupt triggers. Fix this by initializing the
workqueue structures before requesting interrupts.

CRs-Fixed: 410432
Change-Id: Iaace5bc1d0611619f11b760ea6986eadb7346da1
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>