ALSA: line6: add hw monitor volume control to POD HD500X

Add hw monitor volume control for POD HD500X. This is done adding
LINE6_CAP_HWMON_CTL to the capabilities

Signed-off-by: Hans P. Moller <hmoller@uc.cl>
Cc: <stable@vger.kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Link: https://patch.msgid.link/20241003232828.5819-1-hmoller@uc.cl