In TX, check if there are sufficient bytes in the buffer to write a
block to SGPIO. If not, write zeros to SGPIO instead.
In RX, check if there is sufficent space in the buffer to store a block
read from SGPIO. If not, do nothing, which discards the data.
In both of these shortfall cases, the M0 count is not incremented.
This ensures that in TX, old data is never repeated. The M0 will not
resume writing TX samples to SGPIO until the M4 count advances,
indicating new data being ready in the buffer. This fixes bug #180.
Similarly, in RX, old data is never overwritten. The M0 will not resume
writing RX samples to the buffer until the M4 count advances, indicating
new space being available in the buffer.
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