net: microchip: vcap api: Use ERR_CAST() in vcap_decode_rule()

There is a warning reported by coccinelle:

./drivers/net/ethernet/microchip/vcap/vcap_api.c:2399:9-16: WARNING:
ERR_CAST can be used with ri

Use ERR_CAST instead of ERR_PTR + PTR_ERR to simplify the
conversion process.

Signed-off-by: Li Zetao <lizetao1@huawei.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>