drivers/net/pcnet.c: Change debug code to fix build warning

Fix:
pcnet.c: In function 'pcnet_probe':
pcnet.c:247:8: warning: variable 'chipname' set but not used
[-Wunused-but-set-variable]

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