vfio: fix FreeBSD documentation

vfio: fix FreeBSD documentation

On FreeBSD, `rte_vfio_is_enabled()` and `rte_vfio_noiommu_is_enabled()`
API calls will not return error, and will instead return 0. This is
intentional, because the caller of this API does not care whether VFIO
is supported at all, and will instead be interested in whether VFIO is
enabled or not. However, the doxygen comments for these functions state
that they will return an error on FreeBSD, which is incorrect.

Fix the doxygen comment to call out the fact that these
functions are only relevant on Linux, but remove the reference to
returning errors.

Fixes: 279b581c897d ("vfio: expose functions")
Cc: stable@dpdk.org
Signed-off-by: Anatoly Burakov <anatoly.burakov@intel.com>
Acked-by: Chenbo Xia <chenbo.xia@intel.com>