fastboot: Ensure we treat CONFIG_FASTBOOT_BUF_ADDR as long

Otherwise:
drivers/usb/gadget/f_fastboot.c:564:32: warning: format "%lx" expects
argument of type "long unsigned int", but argument 3 has type "unsigned
int" [-Wformat=]

Signed-off-by: Tom Rini <trini@konsulko.com>