psm(4): respect tap_disabled configuration with enabled Extended support

This fixes a bug where, even when hw.psm.tap_enabled=0, touchpad taps
were processed.
tap_enabled has three states: unconfigured, disabled, and enabled (-1, 0, 1).
To respect PR kern/139272, taps are ignored only when explicity disabled.

Submitted by: Ben LeMasurier <ben@crypt.ly> (initial version)
MFC after: 2 weeks