If there is a build setup problem where a device driver has not been
setup for the entropy driver then the call to device_get_binding()
will return a NULL value and the code will continue to use this NULL
value. The result is a hard fault later in code execution.
Note that CONFIG_ASSERT is by default off so one has to turn this
configuration on to catch this problem.
Signed-off-by: David Leach <david.leach@nxp.com>