ixgbe: Fix overwriting of rx_mtrl in ixgbe_ptp_hwtstamp_ioctl

This patch corrects a bug introduced by commit f3444d8b. The rxmtrl value for
the UDP port to timestamp on was moved above the switch statement, but was
overwritten to 0 if the ioctl selected one of the V1 filters.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>