libibumad/umad.c: In get_port, ignore sysctl get rate errors

This can cause ibpanic in ibstat when width is not set properly
as can occur when Ethernet port is connected to InfiniBand fabric.

ibpanic: [8167] main: stat of IB device 'mlx5_0' failed: m

With this change, Rate is displayed as 0 with ibstat for
this scenario.

MFC after: 3 days
Approved by: hselasky (mentor), kib (mentor)
Sponsored by: Mellanox Technologies