hwrng: stm32 - fix maybe uninitialized variable warning

This patch fixes the following warning:
drivers/char/hw_random/stm32-rng.c: In function 'stm32_rng_read':
drivers/char/hw_random/stm32-rng.c:82:19: warning: 'sr' may be used
                                        uninitialized in this function

Reported-by: Sudip Mukherjee <sudip.mukherjee@codethink.co.uk>
Suggested-by: Arnd Bergmann <arnd@arndb.de>
Cc: Daniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: Maxime Coquelin <mcoquelin.stm32@gmail.com>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>