Fix type to prevent implicit type conversion: uint32_t instead of uint16_t

[ChrisMuc]

This pull request fixes an implicit type conversion in ofp_netif.c, where an uint16_t is used instead of an uint32_t.

Background and impact of this bug:
pcap-implementation (in pcap.c) of pktio interface initializes mtu-size with: pcap->mtu = PKTIO_PCAP_MTU_MAX;
whereas PKTIO_PCAP_MTU_MAX is defined as: #define PKTIO_PCAP_MTU_MAX (64 * 1024).
64*1024 = 0x00010000, when casted to uint16_t this is 0.
Using the pcap implementation causes the following error:

I 50 0:585118208 ofp_init.c:438] Slow path threads on core 0
I 52 0:585118208 ofp_ifnet.c:214] Device 'pcap:out=test_out.pcap' addr  02:e9:34:80:73:04
E 52 0:585118208 ofp_ifnet.c:231] odp_pktout_maxlen returned too small value: 0
Error: OFP global init failed.