hwrng: u2fzero - account for high quality RNG

The U2F zero apparently has a real TRNG in it with maximum quality, not
one with quality of "1", which was likely a misinterpretation of the
field as a boolean. So remove the assignment entirely, so that we get
the default quality setting.

In the u2f-zero firmware, the 0x21 RNG command used by this driver is
handled as such [1]:

case U2F_CUSTOM_GET_RNG:
if (atecc_send_recv(ATECC_CMD_RNG,ATECC_RNG_P1,ATECC_RNG_P2,
NULL, 0,
appdata.tmp,
sizeof(appdata.tmp), &res) == 0 )
{
memmove(msg->pkt.init.payload, res.buf, 32);
U2FHID_SET_LEN(msg, 32);
usb_write((uint8_t*)msg, 64);
}
else
{
U2FHID_SET_LEN(msg, 0);
usb_write((uint8_t*)msg, 64);
}

This same call to `atecc_send_recv(ATECC_CMD_RNG,ATECC_RNG_P1,
ATECC_RNG_P2,...)` is then also used in the token's cryptographically
critical "u2f_new_keypair" function, as its rather straightforward
source of random bytes [2]:

int8_t u2f_new_keypair(uint8_t * handle, uint8_t * appid, uint8_t * pubkey)
{
struct atecc_response res;
uint8_t private_key[36];
int i;

watchdog();

if (atecc_send_recv(ATECC_CMD_RNG,ATECC_RNG_P1,ATECC_RNG_P2,
NULL, 0,
appdata.tmp,
sizeof(appdata.tmp), &res) != 0 )
{
return -1;
}

So it seems rather plain that the ATECC RNG is considered to provide
good random numbers.

[1] https://github.com/conorpp/u2f-zero/blob/master/firmware/src/custom.c
[2] https://github.com/conorpp/u2f-zero/blob/master/firmware/src/u2f_atecc.c

Cc: Andrej Shadura <andrew.shadura@collabora.co.uk>
Cc: Jiri Kosina <jkosina@suse.cz>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Acked-by: Andrej Shadura <andrew.shadura@collabora.co.uk>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>