ethernet: aquantia: remove redundant checks on error status

The error status err is initialized as zero and then being checked
several times to see if it is less than zero even when it has not
been updated.  It may seem that the err should be assigned to the
return code of the call to the various *offload_en_set calls and
then we check for failure, however, these functions are void and
never actually return any status.

Since these error checks are redundant we can remove these
as well as err and the error exit label err_exit.

Detected by CoverityScan, CID#1398313 and CID#1398306 ("Logically
dead code")

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Lino Sanfilippo <LinoSanfilippo@gmx.de>
Acked-by: Pavel Belous <pavel.belous@aquantia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>