fsi: master-ast-cf: Mask unused bits in RTAG/RCRC

Then reading the RTAG/RCRC "registers" from the coprocessor after
a command is complete, mask out the top bits, only keep the relevant
bits. Microcode v5 will leave garbage in those top bits as a
result of a performance optimization.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
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