modules: openthread: platform: optimize stack usage in crypto_psa

A PSA crypto operation object can be initialized in multiple
ways according to the documentation. For example,
1. Using a dedicated psa_xxx_operation_init() function that
   returns an initialized object.
2. Using memset() to zero out the operation object.

For some PSA crypto driver implementations, using the first
method causes an excessive stack usage if the operation
object is large and psa_xxx_operation_init() is not inlined.
Instead, it is better to stick to memset() for this purpose.

Signed-off-by: Damian Krolik <damian.krolik@nordicsemi.no>