ath5k: use 3 instead of 0x00000003

reviewed beacon timer initialization with register traces from madwifi: what we
are doing is correct :). one minor fix: use 3 instead of 0x00000003 - it's more
readable.

drivers/net/wireless/ath5k/hw.c:        Changes-licensed-under: ISC

Signed-off-by: Bruno Randolf <bruno@thinktube.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>