fpga: dfl: change data type of feature id to u16

The feature id is stored in a 12 bit field in DFH. So a u16 variable is
enough for feature id.

This patch changes all feature id related places to fit u16.

Signed-off-by: Xu Yilun <yilun.xu@intel.com>
Reviewed-by: Tom Rix <trix@redhat.com>
Acked-by: Wu Hao <hao.wu@intel.com>
Signed-off-by: Moritz Fischer <mdf@kernel.org>