spi: stm32: driver uses reset controller only at init

Remove reset controller device reference from the device private
structure since it is used only at probe time and can be discarded
once used to reset the SPI device.

Signed-off-by: Etienne Carriere <etienne.carriere@st.com>
Signed-off-by: Alain Volmat <alain.volmat@foss.st.com>
Link: https://lore.kernel.org/r/1612551572-495-6-git-send-email-alain.volmat@foss.st.com
Signed-off-by: Mark Brown <broonie@kernel.org>