fsldma: reduce kernel text size

Some of the functions are written in a way where they use multiple reads
and writes where a single read/write pair could suffice. This shrinks the
kernel text size measurably, while making the functions easier to
understand.

add/remove: 0/0 grow/shrink: 1/4 up/down: 4/-196 (-192)
function                                     old     new   delta
fsl_chan_set_request_count                   120     124      +4
dma_halt                                     300     272     -28
fsl_chan_set_src_loop_size                   208     156     -52
fsl_chan_set_dest_loop_size                  208     156     -52
fsl_chan_xfer_ld_queue                       500     436     -64

Signed-off-by: Ira W. Snyder <iws@ovro.caltech.edu>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>