wil6210: fix unsigned cid comparison with >= 0

The comparison of cid >= 0 is always true because cid is of type u8
(8 bits, unsigned).

Fix this by removing such comparison and updating the type of
variable cid to u8 in the caller function.

Addresses-Coverity-ID: 1473079 ("Unsigned compared against 0")
Fixes: b9010f105f21 ("wil6210: add FT roam support for AP and station")
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>