ath6kl: replace do while loop with function helpers on ar6000_avail_ev()

This unwraps the do while loops in favor for function helpers.

Cc: Naveen Singh <nsingh@atheros.com>
Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>