[SCSI] zfcp: Wait for free SBAL during exchange config

When sending a exchange config data command, wait for a free SBAL.
This does not matter during adapter initialization, but this is
required for pulling adapter statistics during high I/O load.

Signed-off-by: Swen Schillig <swen@vnet.ibm.com>
Signed-off-by: Christof Schmitt <christof.schmitt@de.ibm.com>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>