nvme-tcp: Initialize sk->sk_ll_usec only with NET_RX_BUSY_POLL

The access to sk->sk_ll_usec should be hidden behind
CONFIG_NET_RX_BUSY_POLL like the definition of sk_ll_usec.

Put access to ->sk_ll_usec behind CONFIG_NET_RX_BUSY_POLL.

Fixes: 1a9460ce ("nvme-tcp: support simple polling")
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Keith Busch <kbusch@kernel.org>