ixgbe: don't use magic size number to assign ptp_caps.name

Rather than using a magic size number, just use sizeof since that will
work and is more robust to future changes.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>