sched: walt: fix compilation for 32 bit

walt_irq_work_lastq_ws variable is defined as atomic64_t. This
type definition is different for 32 bit and 64 bit. Use
atomic64_xxx API to manipulate this variable.

Change-Id: I1b7e0e962b15fa861cd735f47ce72491d8b9100c
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>