e1000e: PTP lock in e1000e_phc_adjustfreq

Add lock in e1000e_phc_adjfreq to prevent concurrent changes to TIMINCA
and SYSTIMH/L.

Signed-off-by: Todd Fujinaka <todd.fujinaka@intel.com>
Tested-by: Jeff Pieper <jeffrey.e.pieper@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>