tg3: Create timer helper functions

This patch seeks to clean up the timer related code.  It begins by
moving one-time timer setup code from tg3_open() to tg3_init_one().
It then creates a function that encapsulates the code needed to start
the timer.  A tg3_timer_stop() function was added for parity.  Finally,
this patch moves all the timer functions to a more suitable location.

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