USB PL2303 emulation. Send data to SPI and read answer.

Protocol:
    "F - change SPI flags (F f val), f== l-LSBFIRST, b-BR [18MHz/2^(b+1)], p-CPOL, h-CPHA\n"
    "G - get SPI status\n"
    "I - reinit SPI\n"
    "R - software reset\n"
    "S - send data over SPI\n"
    "T - show Tms value\n"

APB2 clock is 18MHz