msm: camera: reqmgr: Avoid freeing subdev twice

The 'l_device' pointer in __cam_req_mgr_destroy_subdev is
set to NULL after freeing but this is done on a
local copy of the variable in stack. This results in
double-free when this function is called again. To avoid
this, pass 'l_device' pointer by reference and assign it
to NULL after freeing.

CRs-Fixed: 3120468
Change-Id: If2dde6f1c702bee26a3c8a68c2f45bafbf0f7cd6
Signed-off-by: Nirmal Abraham <quic_c_nabrah@quicinc.com>