powerpc/fsl_sata: Properly clamp maxio to pessimized size

The CAM 'maxio' is a 'pessimized' size, assuming 4k pages and one page
per segment. Since there are at most 63 segments in a transaction with
this driver, and one would necessarily be the indirect segment marker,
clamp the maxio to the minimum of maxphys (tunable) or (63 - 1) pages
(248k).

MFC after: 3 days