pinctrl: pinconf: remove checks on ops->pin_config_get

ops->pin_config_get() is only used in one specific path that will only be taken
for generic pinconf drivers (ops->is_generic == true) when dumping the pinconf
by using debugfs.

By removing the check in pinconf_check_ops(), let's stop pressuring people to
write a pin_config_get() function that will never be used and so will probably
never be tested.

Removing the check in pinconf_pins_show() allows driver to not implement
pin_config_get() but still get a dump of the pinconf in debugfs by implementing
pin_config_dbg_show().

Finally, not implementing pin_config_get() now results in returning -ENOTSUPP
instead of -EINVAL. While this doesn't have any real impact for now, this feels
more right.

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>