mgmt: ec_host_cmd: stm32_spi: fix rx reload for STM32H7 chips

Disable and enable SPI module before every transaction for STM32H7
chips. It is a recommended way in the STM32 RM. In another case, a first
byte of a transaction is always 0x00.

Signed-off-by: Dawid Niedzwiecki <dawidn@google.com>