ixgbe: Fix PHC loophole allowing misconfiguration of increment register

This patch fixes a potential hole when configuring the cycle counter used to
generate the nanosecond time clock. This clock is based off of the SYSTIME
registers along with the TIMINCA registers. The TIMINCA register determines
the increment to be added to the SYSTIME registers every DMA clock tick. This
register needs to be reconfigured whenever the link-speed changes. However,
the value calculated stays the same when link is down and when link is up.
Misconfiguration can occur if the link status changes due to a reset, which
causes the TIMINCA register to be reset. This reset puts the device in an
unstable state where the SYSTIME registers stop incrementing and the PTP
protocol does not function.

The solution is to double check the TIMINCA value and always reset the value
if the register is zero. This prevents a misconfiguration bug that halts the
PHC.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Acked-by: Don Skidmore <donald.c.skidmore@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>