regulator: stm32-vrefbuf: fix check on ready flag

stm32_vrefbuf_enable() wrongly checks VRR bit: 0 stands for not ready,
1 for ready. It currently checks the opposite.
This makes enable routine to exit immediately without waiting for ready
flag.

Fixes: 0cdbf481

 ("regulator: Add support for stm32-vrefbuf")
Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
Cc: stable@vger.kernel.org