libertas: Use shared constant for rfc1042 header

This is one of the 9 drivers redefining rfc1042_header.

Signed-off-by: Pascal Terjan <pterjan@google.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200523212628.31526-1-pterjan@google.com