USB: usbtouchscreen.c: remove dbg() usage

dbg() was a very old USB-specific macro that should no longer
be used. This patch removes it from being used in the driver
and uses dev_dbg() instead.

CC: Dmitry Torokhov <dmitry.torokhov@gmail.com>
CC: Henrik Rydberg <rydberg@euromail.se>
CC: Rusty Russell <rusty@rustcorp.com.au>
CC: Viresh Kumar <viresh.kumar@st.com>
CC: Armando Visconti <armando.visconti@st.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>