fix: use write instead of read to generate SError

With sha 19297eb7 it was expected that read to invalid device memory
will cause SError in EL3 but it was causing sync exception. Change the
read to write for proper behaviour.

Signed-off-by: Manish Pandey <manish.pandey2@arm.com>
Change-Id: I933aa3a80d40795a339cdf2160ebdcad976ab9e2