ath9k_htc: Fix module unloading issue

The maximum number of packets in a single buffer in
stream mode is 10. The driver currently uses 8 - which
caused stack corruption, in the absence of any kind
of OOB checking.

Fixing this to the correct value of 10 fixes the module
unload issue.

Signed-off-by: Sujith <Sujith.Manoharan@atheros.com>
Tested-by: Ming Lei <tom.leiming@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>