nvme: NVME_TCP_OFFLOAD should not default to m

The help text for the symbol controlling support for the NVM Express
over Fabrics TCP offload common layer suggests to not enable this
support when unsure.

Hence drop the "default m", which actually means "default y" if
CONFIG_MODULES is not enabled.

Fixes: f0e8cb61 ("nvme-tcp-offload: Add nvme-tcp-offload - NVMeTCP HW offload ULP")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: David S. Miller <davem@davemloft.net>