app/testpmd: simplify handling of stats mappings error

app/testpmd: simplify handling of stats mappings error

Simplifiy the logic in the error checking branch. Rather than having a
single error branch which checked both RX and TX conditions and made
extensive use of the ? operator, move the error checking explicitly into
the RX and TX individual branches.

The original code caused compilation issues when attempting compilation
with clang on BSD, due to type mismatches. This change fixes the issues
while making the code easier to read and maintain overall.

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