spi/amba-pl022: fixing compilation warning.

clk_freq is used uninitialized in pl022_setup routine. This patch
fix compilation warning for using uninitialized variable

Signed-off-by: Viresh Kumar <viresh.kumar@st.com>
Acked-by: Linus Walleij <linus.walleij@stericsson.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>