Staging: fix wlan-ng printk format warning

Fix prism2fw.c printk format warning:

drivers/staging/wlan-ng/prism2fw.c:209: warning: format '%d' expects type 'int', but argument 3 has type 'size_t'

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>