net/txgbe: fix build by simplifying xstats return
net/txgbe: fix build by simplifying xstats return

When DPDK is compiled with gcc 7.5 with the optimization level set to 1
gcc sees the 'offset' variable in txgbe_ethdev.c as possibly being
uninitialised.
The 'txgbe_get_offset_by_id()' return value, "-(int)(id + 1)", seems
confusing gcc that it assumes '0' can be returned in the failure case.
To correct this the return statement for error case in
'txgbe_get_offset_by_id()' is simplified to return '-1'.

Fixes: 91fe49c87d76 ("net/txgbe: support device xstats")

Signed-off-by: Conor Walsh <e53367a822c9a2a32150eba4386d4e8c8c837c8e@intel.com>
Reviewed-by: Ferruh Yigit <ba8734c3c506d2e3557bd4ac8fdaccd25bdb9aa7@intel.com>

john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk