nrfmin: fix type for NETOPT_PROTO

The type [is documented as `gnrc_nettype_t`][1] so it should by checked
as such.

[1]: https://github.com/RIOT-OS/RIOT/blob/f9a3bdf1a749f0e6a0d9e79eee652d8b31d4d9e0/sys/include/net/netopt.h#L210-L212