psa: driver wrapper: Fix buffer allocation in case of key generation
psa: driver wrapper: Fix buffer allocation in case of key generation

In case of a secure element or cryptoprocessor with
storage, when generating a key, the key material is
not exported from the secure element or cryptoprocessor
thus there is no need to allocate a buffer in that case.

Signed-off-by: Ronald Cron <b35ff5b7ca26c4ae9b66b92f2cf34224c8e26b08@arm.com>

NXPmicro/mbedtls,NXPmicro/mbedtls,ARMmbed/mbedtls,Mbed-TLS/mbedtls,Mbed-TLS/mbedtls,ARMmbed/mbedtls,NXPmicro/mbedtls,Mbed-TLS/mbedtls,NXPmicro/mbedtls,ARMmbed/mbedtls,Mbed-TLS/mbedtls,ARMmbed/mbedtls