tg3: Replace pr_err with sensible alternatives

All the instances of pr_err() in the tg3 driver have better
alternatives.  This patch replaces all pr_err() calls.

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