sisusb_con: fix coccinelle warning

After commit d705ff38 (tty: vt, cleanup and document con_scroll), in
the coccinelle output, we can see:
drivers/usb/misc/sisusbvga/sisusb_con.c:852:8-9: WARNING: return of 0/1 in function 'sisusbcon_scroll_area' with return type bool

Return true instead of 1 in the function returning bool which was
intended to do in d705ff38 but omitted.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Fixes: d705ff38 (tty: vt, cleanup and document con_scroll)
Cc: Thomas Winischhofer <thomas@winischhofer.net>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: linux-usb@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>