mlx5: Fix wrong size allocation for QoS ETC TC register

The driver allocates wrong size (due to wrong struct name) when issuing
a query/set request to NIC's register.

Linux commit:
d14fcb8d877caf1b8d6bd65d444bf62b21f2070c

Approved by: hselasky (mentor)
MFC after: 1 week
Sponsored by: Mellanox Technologies