ahci: Fix a wrong parameter pass

In stead of user_buffer_size, transfer_size should be used to pass to
ahci_device_data_io(). transfer_size is the length that we want the
low level function to transfer each time.
If we use user_buffer_size which is the totally data length as parameter,
low level function will actually create many SGs to transfer as many data
as possible each time. That will produce many redundant data transfer.

Signed-off-by: Tang Yuantian <Yuantian.Tang@freescale.com>
Reviewed-by: Simon Glass <sjg@chromium.org>