bluetooth: Fix compiler warning with arm-clang

Since we enable -fshort-enums for arm-clang we get the following
warning:

subsys/bluetooth/mesh/pb_adv.c:139:40: warning: cast to smaller
integer type 'enum prov_bearer_link_status' from
 'void *' [-Wvoid-pointer-to-enum-cast]

Fix this by first casting to an int to grow the size of the type.

Signed-off-by: Kumar Gala <kumar.gala@intel.com>