Rather than busy looping, yield back to the scheduler and sleep for a
bit in the event that there's no data. This should hopefully prevent the
stalls that Mark reported:

<6>[    3.362859] Freeing initrd memory: 16196K
<3>[   23.160131] rcu: INFO: rcu_sched self-detected stall on CPU
<3>[   23.166057] rcu:  0-....: (2099 ticks this GP) idle=03b4/1/0x40000002 softirq=28/28 fqs=1050
<4>[   23.174895]       (t=2101 jiffies g=-1147 q=2353 ncpus=4)
<4>[   23.180203] CPU: 0 PID: 49 Comm: hwrng Not tainted 6.0.0 #1
<4>[   23.186125] Hardware name: BCM2835
<4>[   23.189837] PC is at bcm2835_rng_read+0x30/0x6c
<4>[   23.194709] LR is at hwrng_fillfn+0x71/0xf4
<4>[   23.199218] pc : [<c07ccdc8>]    lr : [<c07cb841>]    psr: 40000033
<4>[   23.205840] sp : f093df70  ip : 00000000  fp : 00000000
<4>[   23.211404] r10: c3c7e800  r9 : 00000000  r8 : c17e6b20
<4>[   23.216968] r7 : c17e6b64  r6 : c18b0a74  r5 : c07ccd99  r4 : c3f171c0
<4>[   23.223855] r3 : 000fffff  r2 : 00000040  r1 : c3c7e800  r0 : c3f171c0
<4>[   23.230743] Flags: nZcv  IRQs on  FIQs on  Mode SVC_32  ISA Thumb  Segment none
<4>[   23.238426] Control: 50c5387d  Table: 0020406a  DAC: 00000051
<4>[   23.244519] CPU: 0 PID: 49 Comm: hwrng Not tainted 6.0.0 #1

Link: https://lore.kernel.org/all/Y0QJLauamRnCDUef@sirena.org.uk/Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>