spi/pl022: make the chip deselect handling thread safe

There is a possibility that the pump_message and giveback
run in parallel on a SMP system. Both the pump_message and
giveback threads work on the same SPI message queue.
Results will be in correct if the pump_message gets to
work on the queue first.

when the pump_message works with the queue, it reads the
head of the queue and removes it from the queue. pump_message
activates the chip select depending on this message read.

This leads to giveback working on the modified queue or a
emptied queue. If the queue is empty or if the next message on
the queue (which is not the actual next message, as the pump
message has removed the next message from the queue) is not for
the same SPI device as that Of the previous one, giveback will
de-activate the chip select activated by pump_message(),
which is wrong.

To solve this problem pump_message is made not to run and
access the queue until the giveback is done handling the queue.
I.e. by making the cur_msg NULL after the giveback has read the
queue.

Also a state variable has been introduced to keep track of when
the CS for next message is already activated and avoid to
double-activate it.

Reviewed-by: Viresh Kumar <viresh.kumar@st.com>
Signed-off-by: Virupax Sadashivpetimath <virupax.sadashivpetimath@stericsson.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>