spi: can use a tx threshold of 50%

The TX fifo threshold is pretty arbitrary.
Set this too big and too many interrupts will occur
for no good reason. Set it too small, and latency
in the SPI transactions is introduced. User's will probably
have to tune this per their application and SPI frequency, etc.
I think setting this to 50% is a good guess for now.

Change-Id: Ib325d40bc7ee10473d99443b3b3cd00fd6e4b95f
Signed-off-by: Chuck Jordan <cjordan@synopsys.com>