tg3: Skip powering down function 0 on certain serdes devices

commit 44f3b503c16425c8e9db4bbaa2fc9cd0c9d0ba91 upstream.

On the 5718, 5719 and 5720 serdes devices, powering down function 0
results in all the other ports being powered down. Add code to skip
function 0 power down.

v2:
 - Modify tg3_phy_power_bug() function to use a switch instead of a
   complicated if statement. Suggested by Joe Perches.

Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: Nithin Nayak Sujir <nsujir@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
[bwh: Backported to 3.2:
 s/tg3_asic_rev\(tp\)/GET_ASIC_REV(tp->pci_chip_rev_id)/]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>