ixgbe: use 'cc' instead of 'hw_cc' for local variable

The ixgbe_ptp.c file sometimes uses hw_cc as the local variable for the
cycle counter in ixgbe_ptp_read_X550. However, we use just 'cc' as
a local variable for this by convention else where in the file.

Convert this lone usage of 'hw_cc' into just the shorter 'cc' name to
match the other read functions in the file.

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>