tg3: Add ability to turn off 1shot MSI

In older devices, 1-shot MSI mode had to be enabled by the code.  In
newer devices however, 1-shot MSI mode is enabled by default; code would
be needed to disable it.

Disabling 1-shot MSI mode is useful when debugging.  This patch changes
the code so that the TG3_FLAG_1SHOT_MSI accurately reflects (and
controls) the state of 1-shot MSI mode.

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