dts/arm/st: wl: decrease Sub-GHz SPI frequency to 8MHz

Commit 246ea739 ("dts/arm/st: wl: increase Sub-GHz SPI frequency to
12MHz") increased the Sub-GHz to 12 MHz. This matches the SX126x
datasheet, but there is no information about the maximum speed in the
STM32WL datasheet or reference.

This works fine when not using DMA. However with DMA activated (adding
entries to the device-tree and enabling CONFIG_SPI_STM32_DMA), I have
encountered some rare corruption. When it happens, the read from the
Sub-GHz device gets an extra 0x00 byte prepended, which confuses the
LoRaMac-node library and causes reception failures. Decreasing the
frequency to the next round number, that is 8 MHz (i.e. increasing the
prescaler from 4 to 6) fixes all the issues I encountered.

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>