i40e: use a mutex instead of spinlock in PTP user entry points

We need a locking mechanism to protect the hardware SYSTIME register
which is split over 2 values, and has internal hardware latching. We
can't allow multiple accesses at the same time. However....

The spinlock_t is overkill here, especially use of spin_lock_irqsave,
since every PTP access will halt hardirqs. Notice that the only places
which need the SYSTIME value are user context and are capable of sleeping.
Thus, it is safe to use a mutex here instead of the spinlock.

Change-ID: I971761a89b58c6aad953590162e85a327fbba232
Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>