ath9k_htc: Delete unnecessary variable initialisation

In ath9k_hif_usb_rx_stream(), i is initialised in the for loop it's
used in.

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Reviewed-by: Oleksij Rempel <linux@rempel-privat.de>
[Rewrote commit message]
Signed-off-by: Julian Calaby <julian.calaby@gmail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>