fpga: dfl: make uinit callback optional

This patch makes uinit callback of sub features optional. With
this change, people don't need to prepare any empty uinit callback.

Signed-off-by: Wu Hao <hao.wu@intel.com>
Link: https://lore.kernel.org/r/1564914022-3710-9-git-send-email-hao.wu@intel.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>