hwrng: st - Use real-world device timings for timeout

Samples are documented to be available every 0.667us, so in theory
the 8 sample deep FIFO should take 5.336us to fill.  However, during
thorough testing, it became apparent that filling the FIFO actually
takes closer to 12us.

Also take into consideration that udelay() can behave oddly i.e. not
delay for as long as requested.

  Suggested-by: Russell King <rmk+kernel@arm.linux.org.uk>:

    "IIRC, Linus recommends a x2 factor on delays, especially
     timeouts generated by these functions.
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

drivers/char/hw_random/st-rng.c

@@ -32,8 +32,14 @@
 #define ST_RNG_FIFO_SIZE		8
 #define ST_RNG_SAMPLE_SIZE		2 /* 2 Byte (16bit) samples */
 
-/* Samples are available every 0.667us, which we round to 1us */
-#define ST_RNG_FILL_FIFO_TIMEOUT   (1 * (ST_RNG_FIFO_SIZE / ST_RNG_SAMPLE_SIZE))
+/*
+ * Samples are documented to be available every 0.667us, so in theory
+ * the 4 sample deep FIFO should take 2.668us to fill.  However, during
+ * thorough testing, it became apparent that filling the FIFO actually
+ * takes closer to 12us.  We then multiply by 2 in order to account for
+ * the lack of udelay()'s reliability, suggested by Russell King.
+ */
+#define ST_RNG_FILL_FIFO_TIMEOUT	(12 * 2)
 
 struct st_rng_data {
 	void __iomem	*base;