Fix kernel panics in fastboot on OS X.

The kernel panic seems to be related to the driver trying to allocate
too many pages from the IO mapper.  That may be caused by the fact
that we try to perform a 100+ MiB transfer in a single IO operation.
This change breaks the transfer down into 1 MiB chunks.

So far after a day of testing, no kernel panics have occurred compared
to 5 in the previous 24 hours!

Change-Id: I8d27a546e0c0bf4fe2f0fc7fcad65a88d3e6bee0