phy: stm32: fix st,slow-hs-slew-rate with st,decrease-hs-slew-rate

st,decrease-hs-slew-rate is described in phy-stm32-usbphyc.yaml. Then
fix the property name in driver.

Fixes: 2f5e9f81 ("phy: stm32: add phy tuning support")
Signed-off-by: Amelie Delaunay <amelie.delaunay@foss.st.com>
Link: https://lore.kernel.org/r/20211026154817.198937-1-amelie.delaunay@foss.st.com


Signed-off-by: Vinod Koul <vkoul@kernel.org>