Return the integer value to be stored from the hex string

def get_prep_value(self, value):
		""" Return the integer value to be stored from the hex string """
		if value is None or value == "":
			return None
		if isinstance(value, six.string_types):
			value = _hex_string_to_unsigned_integer(value)
		if _using_signed_storage():
			value = _unsigned_to_signed_integer(value)
		return value