wil6210: accessing 802.3 addresses via utility functions

Rearrange the code by having functions to access 802.3 header
members, source and destination addresses.

Signed-off-by: Ahmad Masri <amasri@codeaurora.org>
Signed-off-by: Maya Erez <merez@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>