crypto: hisilicon/trng - fix local variable type

The return value of 'readl_poll_timeout' is '0' or '-ETIMEDOUT'. Therefore,
change the local variable 'ret' type from 'u32' to 'int'.

Signed-off-by: Weili Qian <qianweili@huawei.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>