nrfmin: fix type for NETOPT_PROTO

[miri64]

Contribution description

The type is documented as gnrc_nettype_t so it should by checked
as such.

Testing procedure

nrfmin should still send 6Lo frames without running into an assertion.

Issues/PRs references

Revealed in #10532 (comment) and thus required for #10532.

[cladmi]

I tested this one alone and also tested it merged with #10532

USEMODULE=nrfmin CFLAGS='-DDEBUG_ASSERT_VERBOSE=1' BOARD=nrf52dk make -C examples/gnrc_networking flash

And I could ping6 two nrf52dk boards with each other for both cases.
So it fixes the issue from #10532 (comment)

[cladmi]

I reviewed the change with @miri64 and the change makes it the same as

RIOT/drivers/netdev_ieee802154/netdev_ieee802154.c

Lines 132 to 135 in 4e5de1b

	case NETOPT_PROTO:
	assert(max_len == sizeof(gnrc_nettype_t));
	*((gnrc_nettype_t *)value) = dev->proto;
	res = sizeof(gnrc_nettype_t);

So looks good for this and it fixes the issue.