wil6210: fix wil_cid_valid with negative cid values

There are several occasions where a negative cid value is passed
into wil_cid_valid and this is converted into a u8 causing the
range check of cid >= 0 to always succeed.  Fix this by making
the cid argument an int to handle any -ve error value of cid.

An example of this behaviour is in wil_cfg80211_dump_station,
where cid is assigned -ENOENT if the call to wil_find_cid_by_idx
fails, and this -ve value is passed to wil_cid_valid.  I believe
that the conversion of -ENOENT to the u8 value 254 which is
greater than wil->max_assoc_sta causes wil_find_cid_by_idx to
currently work fine, but I think is by luck and not the
intended behaviour.

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Maya Erez <merez@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>