wil6210: fix min() compilation errors

With some tool chains compilation fails due to type
mismatch of the arguments passed to min().
Use min_t() to solve these compilation errors.

Signed-off-by: Alexei Avshalom Lazar <ailizaro@codeaurora.org>
Signed-off-by: Maya Erez <merez@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>