USB: belkin_sa.c: remove dbg() usage

dbg() is a usb-serial specific macro.  This patch converts
the belkin_sa.c driver to use dev_dbg() instead to tie into the
dynamic debug infrastructure.

CC: William Greathouse <wgreathouse@smva.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>