drivers/net/wireless/ath9k/rc.c: use ARRAY_SIZE

ARRAY_SIZE is more concise to use when the size of an array is divided by
the size of its type or the size of its first element.

The semantic patch that makes this change is as follows:
(http://www.emn.fr/x-info/coccinelle/)

// <smpl>
@i@
@@

#include <linux/kernel.h>

@depends on i using "paren.iso"@
type T;
T[] E;
@@

- (sizeof(E)/sizeof(E[...]))
+ ARRAY_SIZE(E)
// </smpl>

Signed-off-by: Julia Lawall <julia@diku.dk>
Signed-off-by: John W. Linville <linville@tuxdriver.com>