cxgbei: Wait for socket to close in icl_cxgbei_conn_close.

This ensures the TOE has finished processing any in-flight received
data before returning to the caller. The caller assumes it is safe to
free any open tasks or transfers (and associated buffers) after this
function returns.

Previously, data placed directly via DDP could be written to buffers
after the caller had freed the buffers.

Reported by: Jithesh Arakkan @ Chelsio
Sponsored by: Chelsio Communications