app/testpmd: fix macro check for little endian

app/testpmd: fix macro check for little endian

Compiling with clang on FreeBSD gave a compilation error:
app/test-pmd/csumonly.c:84:5: fatal error: '__BYTE_ORDER' is not defined, evaluates to 0 [-Wundef]

Querying the preprocessor defines show both the define and value used
are incorrect.
	$ clang -dM -E - < /dev/null | grep BYTE
	\#define  __BYTE_ORDER__  __ORDER_LITTLE_ENDIAN__

Changing the check to  __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__ then
resolves the issue.

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