entropy: b91_trng: Fix callback return

get_entropy_isr() has to return the number of bytes copied or a negative
value for error. Since this driver is assuming that it will always
(????) get the number of requested bytes, change the function to return it
instead of 0.

Signed-off-by: Flavio Ceolin <flavio.ceolin@intel.com>

drivers/entropy/entropy_b91_trng.c

@@ -55,7 +55,7 @@ static int entropy_b91_trng_get_entropy_isr(const struct device *dev,
 	/* No specific handling in case of running from ISR, just call standard API */
 	entropy_b91_trng_get_entropy(dev, buffer, length);
 
-	return 0;
+	return length;
 }
 
 /* Entropy driver APIs structure */