i40e: fix build with 16-byte descriptors

i40e: fix build with 16-byte descriptors

The compile error will occur as below when set 'RTE_LIBRTE_I40E_16BYTE_RX_DESC=y'.
'fd_id' should be used to replace 'fd', as 'fd' is not defined in that structure
at all. In addition, local variable of 'flexbl' and 'flexbh' must be used only if
32 bytes RX descriptor is selected.

error logs:
lib/librte_pmd_i40e/i40e_rxtx.c: In function i40e_rxd_build_fdir:
lib/librte_pmd_i40e/i40e_rxtx.c:431:28: error: volatile union <anonymous> has no member named fd
lib/librte_pmd_i40e/i40e_rxtx.c:427:19: error: unused variable flexbl [-Werror=unused-variable]
lib/librte_pmd_i40e/i40e_rxtx.c:427:11: error: unused variable flexbh [-Werror=unused-variable]

Signed-off-by: Helin Zhang <helin.zhang@intel.com>
Acked-by: Jingjing Wu <jingjing.wu@intel.com>