e1000e: Fix 82572EI that has no hardware timestamp support

With the Intel 82527EI (driver: e1000e) there is an issue when running
the ptpd2 program, that leads to a kernel oops.  The reason is here that
in e1000_xmit_frame() a work queue will be scheduled that has not been
initialized in this case.  The work queue "tx_hwstamp_work" will only be
initialized if adapter->flags & FLAG_HAS_HW_TIMESTAMP set.  This check
is missing in e1000_xmit_frame().

The following patch adds the missing check.

Signed-off-by: Mathias Koehrer <mathias.koehrer@etas.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>