USB-Audio: Solid State Labs SSL 2+ - fix capture channels

USB-Audio: Solid State Labs SSL 2+ - fix capture channels

It seems that there are 4 capture channels per report in issue #552:

    Capture:
      Status: Stop
      Interface 2
        Altset 1
        Format: S32_LE
        Channels: 4
        Endpoint: 0x81 (1 IN) (ASYNC)
        Rates: 44100, 48000, 88200, 96000, 176400, 192000
        Data packet interval: 125 us
        Bits: 24
        Channel map: FL FR FC LFE

Closes: https://github.com/alsa-project/alsa-ucm-conf/issues/552
Signed-off-by: Jaroslav Kysela <perex@perex.cz>