rtl8139: use TARGET_FMT_plx in debug messages

Prevents a compilation failure when DEBUG_RTL8139 is defined:

CC    libhw32/rtl8139.o
cc1: warnings being treated as errors
hw/rtl8139.c: In function ‘rtl8139_cplus_transmit_one’:
hw/rtl8139.c:1960: error: format ‘%8lx’ expects type ‘long unsigned int’, but argument 5 has type ‘target_phys_addr_t’
make[1]: *** [rtl8139.o] Error 1

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>
