ath9k_htc: Increase URB count for REG_IN pipe

Using a single URB for receiving WMI events is
insufficient, increase it to 64 to not lose
WMI events in high throughput situations.

Signed-off-by: Sujith Manoharan <Sujith.Manoharan@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>