cam-service: Simplify code shared between Zephyr and Linux

Zephyr requires the explicit initialization of pthread attr to be able
to set the thread's stack area. Whilst in Linux we can rely on the
implicit initialization of the pthread attr, it only makes the code less
readable.

This patch keeps the explicit initialization of pthread attr for Linux
as well.

Signed-off-by: Filipe Rinaldi <filipe.rinaldi@arm.com>