EAP-GPSK: Clean up CSuite_List length validation (CID 62854)

EAP-GPSK: Clean up CSuite_List length validation (CID 62854)

Use a local variable and size_t in length comparison to make this easier
for static analyzers to understand. In addition, set the return list and
list_len values at the end of the function, i.e., only in success case.
These do not change the actual behavior of the only caller for this
function, but clarifies what the helper function is doing.

Signed-off-by: Jouni Malinen <j@w1.fi>