qcacmn: Use size_t for qdf_str_len() return type

A recent commit changed the return type for qdf_str_len() to size_t from
int32_t. Update call sites for qdf_str_len() to store the return value
as size_t to match.

Change-Id: Ib8bfad2c1fc7de1f6fc601d1be69e734d3a49dcf
CRs-Fixed: 2196858
2 files changed