net/mlx5e: Fix select queue to consider SKBTX_HW_TSTAMP

Steering packets to PTP-SQ should be done only if the SKB has
SKBTX_HW_TSTAMP set in the tx_flags. While here, take the function into
a header and inline it.
Set the whole condition to select the PTP-SQ to unlikely.

Fixes: 24c22dd0

 ("net/mlx5e: Add states to PTP channel")
Signed-off-by: Aya Levin <ayal@nvidia.com>
Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
Reviewed-by: Maxim Mikityanskiy <maximmi@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>