Set STM32_MAC_DMABMR_SR to make ethernet work again

ChibiOS 2.6.4 introduced the option to enable or disable setting
the DMABMR SR (software reset) bit, where it was always doing this
before. The new default behavior is to not set it, and this was
causing eth_mii_start to hang trying to clear the TX fifo (ETH_DMAOMR_FTF).