net: mana: Fix the tso_bytes calculation

sizeof(struct hop_jumbo_hdr) is not part of tso_bytes, so remove
the subtraction from header size.

Cc: stable@vger.kernel.org
Fixes: bd7fc6e1

 ("net: mana: Add new MANA VF performance counters for easier troubleshooting")
Signed-off-by: Haiyang Zhang <haiyangz@microsoft.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Reviewed-by: Shradha Gupta <shradhagupta@linux.microsoft.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>