ibmvscsis: Fix max transfer length

Current code incorrectly calculates the max transfer length, since
it is assuming a 4k page table, but ppc64 all run on 64k page tables.

Cc: stable@vger.kernel.org
Reported-by: Steven Royer <seroyer@linux.vnet.ibm.com>
Tested-by: Steven Royer <seroyer@linux.vnet.ibm.com>
Signed-off-by: Bryant G. Ly <bryantly@linux.vnet.ibm.com>
Signed-off-by: Bart Van Assche <bart.vanassche@sandisk.com>