ALSA: hda - Fix silent headphone output on Panasonic CF-74

CF-74 does the headphone/speaker switching on hardware, thus the driver
shouldn't do any software-toggling of pins.  Otherwise it results in a
silent headphone output.

This patch simply resets the hp_detect flag to fix the problem.

Signed-off-by: Takashi Iwai <tiwai@suse.de>