msm: camera: core: Allocate memory for flush req array dynamically

Allocate memory for flush req array dynamically based
on the list count, in case the req count bigger than
the size of req list.

CRs-Fixed: 2835525
Change-Id: I7e48fb9dc7815cb991c84195e761a3c3f1c26b48
Signed-off-by: Depeng Shao <depengs@codeaurora.org>