Probably unnecessary, but feeling paranoid about changing sample rates while there's a processor actively handling samples, and potentially maxing out the M4 core.
Addresses long-standing and annoying bug where SGPIO DMA channel would not disable -- and not configure cleanly next time it was needed. My theory is that the DMA channel couldn't disable until it got a request from the peripheral, and sometimes the peripheral was disabled before that last request.
Anyway, the baseband firmware should control the SGPIO, methinks, despite the impact on baseband code size.
Turns out 0x10088000 - 0x10089fff is not the right place. It's 0x40041000, and is only 256 bytes!
Move PPM to correct place, change representation to PPB for finer control.
Reset PPB value to initial value if read value out of bounds.
Clip PPB value on write to permitted bounds.
Contributes to resolution of issue #11.
