tg3: phy tmp variable roundup

The tg3's phy routines define temporary variables in many locations
within the same routine.  This patch unifies all temporary variables
into one location.

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