amixer: fix indentation when printing container TLV contents

amixer: fix indentation when printing container TLV contents

decode_tlv() adds indentation in the beginning, with the expectation
that the TLV will be printed on the line afterwards in a switch-case.

However, in the case of a container TLV the switch-case simply adds
another level of indentation and calls decode_tlv() for the inner TLVs.
This causes the first inner TLV to be printed with too much indentation
and double '|'.

Fix that by printing "container" and a newline for container TLVs, so
that the result is as follows:
  : values=0,0,0,0,0,0,0,0
  | container
    | chmap-variable=FL,FR
    | chmap-variable=FL,FR,LFE
    | chmap-variable=FL,FR,FC

Signed-off-by: Anssi Hannula <anssi.hannula@iki.fi>
Signed-off-by: Takashi Iwai <tiwai@suse.de>