net: chelsio: Add a missing check on cudg_get_buffer

cudbg_collect_hw_sched() could fail when the function cudg_get_buffer()
returns an error. The fix adds a check to the latter function returning
error on failure

Signed-off-by: Aditya Pakki <pakki001@umn.edu>
Signed-off-by: David S. Miller <davem@davemloft.net>