net/bnxt: fix stats errors handling

net/bnxt: fix stats errors handling

This patch fixes few checks and few return values while getting
and clearing device statistics.

1. Fixed to return standard error code.
2. Clubbed few error checks
3. Removed an unnecessary return check

Fixes: bfb9c2260be2 ("net/bnxt: support xstats get/reset")
Fixes: 88920136688c ("net/bnxt: support xstats get by id")
Cc: stable@dpdk.org
Signed-off-by: Kalesh AP <kalesh-anakkur.purayil@broadcom.com>
Reviewed-by: Somnath Kotur <somnath.kotur@broadcom.com>
Reviewed-by: Ajit Khaparde <ajit.khaparde@broadcom.com>