ath6kl: increase usb rx buffer size to 4096

With the previous value (1700), some urb are dropped
with a babble error (urb status equal -EOVERFLOW).

These error seems to only happen when urb length is a
multiple of packet size (512).

Signed-off-by: Julien Massot <jmassot@aldebaran-robotics.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>