drivers/net/wireless/ath/ath6kl/hif.c: drop if around WARN_ON

Just use WARN_ON rather than an if containing only WARN_ON(1).

A simplified version of the semantic patch that makes this transformation
is as follows: (http://coccinelle.lip6.fr/)

// <smpl>
@@
expression e;
@@
- if (e) WARN_ON(1);
+ WARN_ON(e);
// </smpl>

Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>