staging: ath6kl: Add configuration for excessive TX retry threshold

Adding host side interface to configure the excessive TX retry threshold.
It is used by the target to determine disconnection triggers. Additionally,
some definitions have been added to header file wmi.h to bridge the gap
for the newly added command.

Signed-off-by: Vipin Mehta <vmehta@atheros.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>