spi/pxa2xx: Modify RX-Tresh instead of busy-loop for the remaining RX bytes.

After all TX bytes are sent, the driver spins while the SPI core is busy
and then it spins for a "short" period of time until RX bytes are
available.
On Sodavile the busy flag disappears pretty quick and after that it
takes approx ~130ms (sometimes less but not much) until there are bytes
available in the RX FIFO.
This patch removes the busy loop and modifies the RX threshould so we
get woken up once the remainings bytes arrived.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Dirk Brandewie <dirk.brandewie@gmail.com>