bnx2x: Disable HC coalescing when setting timeout to zero.

Problem reported by Flavio Leitner <fleitner@redhat.com>:
When setting rx/tx coalescing timeout to the values less than 12 traffic was
stopped.

The FW supports coalescing in 12us granularity, and so value of less then 12
should be interpreted as disabling coalescing

Signed-off-by: Vlad Zolotarov <vladz@broadcom.com>
Signed-off-by: Eilon Greenstein <eilong@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>