rng: fix clock error handling, based on RM.

According to L4/L0/G0 RM, in case of clock error, interrupt flag must
be cleared, and CECS flag should be cleared as soon as clock meets
requirement.

Reviewed-on: https://github.com/libopencm3/libopencm3/pull/1062

lib/stm32/common/rng_common_v1.c

@@ -87,6 +87,9 @@ bool rng_get_random(uint32_t *rand_nr)
  * Get a random number and block until it works.
  * Unless you have a clock problem, this should always return "promptly"
  * If you have a clock problem, you will wait here forever!
+ * Check device RM for clock requirements (usually fRNGCLK > fHCLK/16 or 
+ * fRNGCLK > fHCLK/32
+
  * @returns a random 32bit number
  */
 uint32_t rng_get_random_blocking(void)
@@ -104,6 +107,10 @@ uint32_t rng_get_random_blocking(void)
                         RNG_CR |= RNG_CR_RNGEN;
                 }
 
+                if (RNG_SR & RNG_SR_CEIS) {
+                        RNG_SR = RNG_SR & ~RNG_SR_CEIS;
+                }
+
                 done = rng_get_random(&rv);
         } while (!done);