wil6210: Delete an unnecessary kfree() call in wil_tid_ampdu_rx_alloc()

A null pointer would be passed to a call of the function “kfree”
directly after a call of the function “kcalloc” failed at one place.
Remove this superfluous function call.

This issue was detected by using the Coccinelle software.

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Reviewed-by: Maya Erez <merez@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>