spi: microchip-core: ensure TX and RX FIFOs are empty at start of a transfer

In the Linux kernel, the following vulnerability has been resolved: spi: microchip-core: ensure TX and RX FIFOs are empty at start of a transfer While transmitting with rx_len == 0, the RX FIFO is not going to be emptied in the interrupt handler. A subsequent transfer could then read crap from the previous transfer out of the RX FIFO into the start RX buffer. The core provides a register that will empty the RX and TX FIFOs, so do that before each transfer.