tg3: Fix for diasllow rx coalescing time to be 0

tg3: Fix for diasllow rx coalescing time to be 0

When the rx coalescing time is 0, interrupts
are not generated from the controller and rx path hangs.
To avoid this rx hang, updating the driver to not allow
rx coalescing time to be 0.

Signed-off-by: Satish Baddipadige <satish.baddipadige@broadcom.com>
Signed-off-by: Siva Reddy Kallam <siva.kallam@broadcom.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>