app/testpmd: fix out-of-range error on bsd

app/testpmd: fix out-of-range error on bsd

The definition value for IPPROTO_DIVERT protocol uses a value
which is out of range of the uint8_t type, giving clang compiler
errors on FreeBSD.

app/test-pmd/icmpecho.c:231:7: fatal error: overflow converting case value
        to switch condition type (258 to 2) [-Wswitch]
                case IPPROTO_DIVERT: /**< divert pseudo-protocol */

This is fixed by having the code to return the protocol name
use the uint16_t type for the protocol value input.

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
Acked-by: Thomas Monjalon <thomas.monjalon@6wind.com>