pc-bios/s390-ccw: support all virtio block size

The block size value may be given "as is" OR as a base value and
a shift count (exponent). So, we have to use calculation to get
the proper number in the code.

The main expression reads as
        (blk_cfg.blk_size << blk_cfg.physical_block_exp)

E.g., various combinations between blk_size=1/physical_block_exp=12
and blk_size=4096/physical_block_exp=0 are valid for 4K blocks.

Signed-off-by: Eugene (jno) Dvurechenski <jno@linux.vnet.ibm.com>
Acked-by: Christian Borntraeger <borntraeger@de.ibm.com>
Reviewed-by: David Hildenbrand <dahi@linux.vnet.ibm.com>
Signed-off-by: Jens Freimann <jfrei@linux.vnet.ibm.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
