ena: fix FreeBSD build

ena: fix FreeBSD build

FreeBSD was not defined in ena_plat.h
ETIME is not defined in FreeBSD.

In file included from DPDK/drivers/net/ena/base/ena_com.h:37:0,
                 from DPDK/drivers/net/ena/ena_ethdev.h:39,
                 from DPDK/drivers/net/ena/ena_ethdev.c:41:
DPDK/drivers/net/ena/base/ena_plat.h:48:2: error: #error "Invalid platform"

Fixes: 99ecfbf845b3 ("ena: import communication layer")
Fixes: 9ba7981ec992 ("ena: add communication layer for DPDK")

Signed-off-by: Daniel Mrzyglod <danielx.t.mrzyglod@intel.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>