dca: make function dca_common_get_tag static

Function dca_common_get_tag is local to the source and does not need to be
in global scope, so make it static.

Cleans up sparse warning:
drivers/dca/dca-core.c:273:4: warning: symbol 'dca_common_get_tag' was
not declared. Should it be static?

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: David S. Miller <davem@davemloft.net>