Remove usage of Mbed TLS internal function from DeviceKey.
Remove usage of Mbed TLS internal function from DeviceKey.

DeviceKey was using mbedtls_ssl_safer_memcmp() to compare result against
buffer of equal length, but zero content. This comparison makes no sense
as the entropy function is already returning proper error, if it fails.

andcor02/mbed-os,mbedmicro/mbed,kjbracey-arm/mbed,kjbracey-arm/mbed,kjbracey-arm/mbed,andcor02/mbed-os,andcor02/mbed-os,mbedmicro/mbed,kjbracey-arm/mbed,mbedmicro/mbed,mbedmicro/mbed,andcor02/mbed-os,mbedmicro/mbed,andcor02/mbed-os,andcor02/mbed-os