rt2x00: use helper to check capability/requirement

Use rt2x00_has_cap_flag macro to check rt2x00dev->cap_flags.

Signed-off-by: Fred Chou <fred.chou.nd@gmail.com>
Acked-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>