fix a fencepost error in the transition from lba28 to lba48
commands.  since drive idents return max lba + 1 and since
0 sectors is valid for removable media, the all ones lba is
not addressable.  therefore the maximum lba28 address
is (1<<28) - 2, not (1<<28) - 1.  many thanks to
squeek@gmail.com for reporting a problem and verifying
the fix.

cosmetically, the useless masks were removed from the
outbs.