psa: Unused key_bits is OK
psa: Unused key_bits is OK

When MD or CMAC are disabled, let the compiler know that it is OK that
`key_bits` is set but not used by casting `key_bits` to `(void)`.

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