tg3: Refine TSO and MSI discovery

This patch consolidates the TSO capability discovery code into its own
code block.  The code that decides whether or not to allow TSO is then
cleaned up.  Finally, the patch consolidates all MSI and MSIX
capability code into a single code block.

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