firmware: cs_dsp: Remove redundant download buffer allocator

Now that cs_dsp uses regmap_raw_write() instead of regmap_raw_write_async()
it doesn't need to keep multiple DMA-safe buffers of every chunk of data
it wrote.

See commit fe08b7d5085a ("firmware: cs_dsp: Remove async regmap writes")

Only one write can be in progress at a time, so one DMA-safe buffer can be
re-used. The single DMA-safe buffer is reallocated if the next write chunk
is larger. Reallocation size is rounded up to reduce the amount of churn.
PAGE_SIZE is used as a convenient size multiple. Typically for .wmfw files
the first chunk is the largest.

A DMA-safe intermediate buffer is used because we can't assume that the
bus underlying regmap can accept non-DMA-safe buffers.

Note that a chunk from the firmware file cannot simply be split into
arbitrarily-sized chunks to avoid buffer reallocation. The data in the
firmware file is preformatted to be written directly to the device as one
block. It already takes account of alignment, write-bursts and write
length requirements of the target hardware, so that the cs_dsp driver can
treat it as an opaque blob.

Signed-off-by: Richard Fitzgerald <rf@opensource.cirrus.com>
Link: https://patch.msgid.link/20251126131501.884188-1-rf@opensource.cirrus.com
Signed-off-by: Mark Brown <broonie@kernel.org>

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