net: qlcnic: clean up sysfs error codes

Replace confusing QL_STATUS_INVALID_PARAM == -1 == -EPERM with -EINVAL
and QLC_STATUS_UNSUPPORTED_CMD == -2 == -ENOENT with -EOPNOTSUPP, the
latter error code is arguable, but it is already used in the driver,
so let it be here as well.

Also remove always false (!buf) check on read(), the driver should
not care if userspace gets its EFAULT or not.

Signed-off-by: Vladimir Zapolskiy <vz@mleia.com>
Acked-by: Rajesh Borundia <rajesh.borundia@qlogic.com>
Signed-off-by: David S. Miller <davem@davemloft.net>