spi/pl022: fix build warnings

The driver build complains with newer compilers unless you
initialize this struct properly.

Reviewed-by: Viresh Kumar <viresh.kumar@st.com>
Signed-off-by: Jonas Aaberg <jonas.aberg@stericsson.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>