wil6210: generalize tx desc mapping

Introduce enum to describe mapping type; allow 'none' in addition to
'single' and 'page'; this is preparation for GSO

Signed-off-by: Vladimir Kondratiev <qca_vkondrat@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
Git-commit: 2232abd59ae5801b20c1e8269a63515bac50d28d
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


Signed-off-by: Liron Kuch <lkuch@codeaurora.org>