tg3: Fix for disallow tx coalescing time to be 0

The recent commit 087d7a8c "tg3: Fix for diasllow rx coalescing
time to be 0" disallow to set Rx coalescing time to be 0 as this stops
generating interrupts for the incoming packets. I found the zero
Tx coalescing time stops generating interrupts for outgoing packets
as well and fires Tx watchdog later. To avoid this, don't allow to set
Tx coalescing time to 0 and also remove subsequent checks that become
senseless.

Cc: satish.baddipadige@broadcom.com
Cc: siva.kallam@broadcom.com
Cc: michael.chan@broadcom.com
Signed-off-by: Ivan Vecera <ivecera@redhat.com>
Acked-by: Siva Reddy Kallam <siva.kallam@broadcom.com>
Acked-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>