spi: pl022: don't use uninitialized variable

The num-cs property is a required property according to the binding
documentation.  However, if it is not present, the driver currently
simply uses random junk from the stack for the num-cs since the variable
whose pointer is passed to of_property_read_u32() is not initialized.

Signed-off-by: Rabin Vincent <rabinv@axis.com>
Signed-off-by: Mark Brown <broonie@kernel.org>