isdn: mISDN, don't compile unused stuff

Remove these compiler warnings:

drivers/isdn/hardware/mISDN/w6692.c:534: warning: `setvolume' defined but not used
drivers/isdn/hardware/mISDN/w6692.c:561: warning: `enable_pots' defined but not used

by moving the functions inside #if 0 ...  #endif.  And an alternative is
to remove them completely if nobody has plans to use them.

Signed-off-by: Jiri Slaby <jirislaby@gmail.com>
Cc: Karsten Keil <isdn@linux-pingi.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: David S. Miller <davem@davemloft.net>