tg3: Cleanup interrupt setup / teardown

Later patches will be adding MSIX support, which will complicate
interrupt initialization.  This patch prepares for the integration by
breaking out the interrupt setup and teardown code into separate
functions and cleaning up the error return paths.

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