cxgb4: fix -ve error check on a signed iq

iq is unsigned, so the error check for iq < 0 has no effect so errors
can slip past this check.  Fix this by making iq signed and also
get_filter_steerq return a signed int so a -ve error can be returned.

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: David S. Miller <davem@davemloft.net>