uboot-imx

net: fastboot: Fix build when FASTBOOT_FLASH is disabled

When building without FASTBOOT_FLASH we don't include the intermediate
update callback to keep the client alive, so ensure we don't try setting
it here.

Signed-off-by: Alex Kiernan <alex.kiernan@gmail.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>