rtc: optee: remove unnecessary memory operations

Remove memcpy by using directly the shared memory.
Remove memset be initialize variable to 0 on stack.

Signed-off-by: Clément Le Goffic <clement.legoffic@foss.st.com>
Link: https://lore.kernel.org/r/20250715-upstream-optee-rtc-v1-2-e0fdf8aae545@foss.st.com


Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>