net/mlx5e: Tidy up IPsec NAT-T SA discovery

IPsec NAT-T packets are UDP encapsulated packets over ESP normal ones.
In case they arrive to RX, the SPI and ESP are located in inner header,
while the check was performed on outer header instead.

That wrong check caused to the situation where received rekeying request
was missed and caused to rekey timeout, which "compensated" this failure
by completing rekeying.

Fixes: d6595493

 ("net/mlx5e: Support IPsec NAT-T functionality")
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>