mwifiex: drop most magic numbers from mwifiex_process_tdls_action_frame()

mwifiex: drop most magic numbers from mwifiex_process_tdls_action_frame()

Before commit 1e58252e334d ("mwifiex: Fix heap overflow in
mmwifiex_process_tdls_action_frame()"),
mwifiex_process_tdls_action_frame() already had too many magic numbers.
But this commit just added a ton more, in the name of checking for
buffer overflows. That seems like a really bad idea.

Let's make these magic numbers a little less magic, by
(a) factoring out 'pos[1]' as 'ie_len'
(b) using 'sizeof' on the appropriate source or destination fields where
    possible, instead of bare numbers
(c) dropping redundant checks, per below.

Regarding redundant checks: the beginning of the loop has this:

                if (pos + 2 + pos[1] > end)
                        break;

but then individual 'case's include stuff like this:

 			if (pos > end - 3)
 				return;
 			if (pos[1] != 1)
				return;

Note that the second 'return' (validating the length, pos[1]) combined
with the above condition (ensuring 'pos + 2 + length' doesn't exceed
'end'), makes the first 'return' (whose 'if' can be reworded as 'pos >
end - pos[1] - 2') redundant. Rather than unwind the magic numbers
there, just drop those conditions.

Fixes: 1e58252e334d ("mwifiex: Fix heap overflow in mmwifiex_process_tdls_action_frame()")
Signed-off-by: Brian Norris <briannorris@chromium.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>