tg3: Add common function tg3_ape_event_lock()

by refactoring code in tg3_ape_send_event().  The common function will
be used in subsequent patches.

Signed-off-by: Matt Carlson <mcarlson@broadcom.com>
Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>