rtl8139: add format attribute to DPRINTF

gcc can check the format string for correctness even when debugging output is
not enabled.
Have to make sure arguments are always available. They are optimized out if
unneeded.

Signed-off-by: Benjamin Poirier <benjamin.poirier@gmail.com>
Cc: Igor V. Kovalenko <igor.v.kovalenko@gmail.com>
Reviewed-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
