USB: serial: mos7840.c: remove dbg() usage

dbg() was a very old USB-serial-specific macro.
This patch removes it from being used in the
driver and uses dev_dbg() instead.

CC: Johan Hovold <jhovold@gmail.com>
CC: Mark Ferrell <mferrell@uplogix.com>
CC: Donald Lee <donald@asix.com.tw>
CC: Tony Zelenoff <antonz@parallels.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>