spi: dw: Fix Tx and Rx threshold after each push/pull

Reading: As when setting up the transfer, Rx has to adapt to current
left Tx lenght.
Writing: If nothing will be transmitted anymore, downsizing the level to
0. This fixes a hanging issue while making the controller being busy for
nothing. Another hack found to fix the same issue was to test the SR
Busy flag bit in the ISR handler. As the threshold level makes more
sense, kepping this one.

Change-Id: I87ba393d507c9418295f188d866d9979f423536c
Signed-off-by: Tomasz Bursztyka <tomasz.bursztyka@linux.intel.com>