drivers/net/rtl8169.c: Fix GCC 4.6 build warning

Fix:
rtl8169.c: In function 'rtl_init':
rtl8169.c:742:13: warning: variable 'printed_version' set but not used
[-Wunused-but-set-variable]

Signed-off-by: Wolfgang Denk <wd@denx.de>