alsa-mixer: set PCM Capture Source for iec958 input

It was reported that on a certain USB card, identified as
"0d8c:0102 C-Media Electronics, Inc. CM106 Like Sound Device",
the "PCM Capture Source" element had to be set to "IEC958 In" before
the iec958 input would work.

The iec958-stereo-input.conf file didn't exist before, although the path
was referenced in the default.conf profile configuration file.

BugLink: https://bugs.freedesktop.org/show_bug.cgi?id=101973

src/modules/alsa/mixer/paths/iec958-stereo-input.conf

@@ -0,0 +1,20 @@
+# This file is part of PulseAudio.
+#
+# PulseAudio is free software; you can redistribute it and/or modify
+# it under the terms of the GNU Lesser General Public License as
+# published by the Free Software Foundation; either version 2.1 of the
+# License, or (at your option) any later version.
+#
+# PulseAudio is distributed in the hope that it will be useful, but
+# WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+# General Public License for more details.
+#
+# You should have received a copy of the GNU Lesser General Public License
+# along with PulseAudio; if not, see <http://www.gnu.org/licenses/>.
+
+[Element PCM Capture Source]
+enumeration = select
+
+[Option PCM Capture Source:IEC958 In]
+name = iec958-input