wic: Make _ptable_format public

wic: Make _ptable_format public

Names with one leasding underscore considered protected in Python.
_ptable_format is accessed outside of its class.
Made it public by removing underscore.

This pylint warning should be fixed now:
Access to a protected member _ptable_format of a client class

Signed-off-by: Ed Bartosh <ed.bartosh@linux.intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>