treewide: rename BIAS_AS_IS to BIAS_UNKNOWN

treewide: rename BIAS_AS_IS to BIAS_UNKNOWN

When inspecting the current bias setting of a GPIO line, the AS_IS name
of one of the possible values really means that the kernel GPIO subsystem
can't determine the bias setting because it didn't set it itself (e.g.
the hardware may have internally initialized pull-up or pull-down
resistors). In this case it's better to change the name to BIAS_UNKNOWN
to reflect that.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>