net/mlx5e: Removed useless code in function

Comparison of eth_ft->ft with NULL is useless, because
get_flow_table() returns either pointer 'eth_ft'
such that eth_ft->ft != NULL, or an erroneous value that is
handled on return, causing mlx5e_ethtool_flow_replace()
to terminate before checking whether eth_ft->ft equals NULL.

Found by Linux Verification Center (linuxtesting.org) with SVACE.

Signed-off-by: Rustam Subkhankulov <subkhankulov@ispras.ru>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>