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 <conor.walsh@intel.com>
Reviewed-by: Ferruh Yigit <ferruh.yigit@intel.com>