wil6210: consider SNAP header in MTU calculations

When encapsulating 802.3 frames into the 802.11 ones,
8-byte SNAP header added to save ethtype. SNAP is part of
the frame body, thus should be counted in MSDU. So,
MTU = MSDU - SNAP

Change-Id: I0ca8225650bd66fab9372ee5d7ed2534a6ed8664
Signed-off-by: Vladimir Kondratiev <qca_vkondrat@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Git-commit: 4590d8125eef6b0c69da71cb1738afbef0100f50
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/linville/wireless-testing.git


Signed-off-by: Hamad Kadmany <hkadmany@codeaurora.org>