gpiolib: Fix debug messaging in gpiod_find_and_request()

When consolidating GPIO lookups in ACPI code, the debug messaging
had been reworked that the user may see

  [   13.401147] (NULL device *): using ACPI '\_SB.LEDS.led-0' for '(null)' GPIO lookup
  [   13.401378] gpio gpiochip0: Persistence not supported for GPIO 40
  [   13.401402] gpio-40 (?): no flags found for (null)

instead of

  [   14.182962] gpio gpiochip0: Persistence not supported for GPIO 40
  [   14.182994] gpio-40 (?): no flags found for gpios

The '(null)' parts are less informative and likely scare the users.
Replace them by '(default)' which can point out to the default connection
IDs, such as 'gpios'.

While at it, amend other places where con_id is used in the messages.

Reported-by: Ferry Toth <ftoth@exalondelft.nl>
Fixes: 8eb1f71e ("gpiolib: consolidate GPIO lookups")
Suggested-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Tested-by: Ferry Toth <ftoth@exalondelft.nl>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>