net: airoha: Fix dev->dsa_ptr check in airoha_get_dsa_tag()

net: airoha: Fix dev->dsa_ptr check in airoha_get_dsa_tag()

Fix the following warning reported by Smatch static checker in
airoha_get_dsa_tag routine:

drivers/net/ethernet/airoha/airoha_eth.c:1722 airoha_get_dsa_tag()
warn: 'dp' isn't an ERR_PTR

dev->dsa_ptr can't be set to an error pointer, it can just be NULL.
Remove this check since it is already performed in netdev_uses_dsa().

Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Closes: https://lore.kernel.org/netdev/Z8l3E0lGOcrel07C@lore-desk/T/#m54adc113fcdd8c5e6c5f65ffd60d8e8b1d483d90
Fixes: af3cf757d5c9 ("net: airoha: Move DSA tag in DMA descriptor")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20250306-airoha-flowtable-fixes-v1-1-68d3c1296cdd@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>