isdn: rename capi_ctr_reseted() to capi_ctr_down()

Change the name of the Kernel CAPI exported function capi_ctr_reseted()
to something representing its purpose better.

Impact: renaming, no functional change
Signed-off-by: Tilman Schmidt <tilman@imap.cc>
Signed-off-by: David S. Miller <davem@davemloft.net>