spi: Create a helper to derive adaptive timeouts

Big transfers might take a bit of time, too constraining timeouts might
lead to false positives. In order to simplify the drivers work and with
the goal of factorizing code in mind, let's add a helper that can be
used by any spi controller driver to derive a relevant per-transfer
timeout value.

The logic is simple: we know how much time it would take to transfer a
byte, we can easily derive the total theoretical amount of time involved
for each transfer. We multiply it by two to have a bit of margin and
enforce a minimum of 500ms.

Suggested-by: Mark Brown <[email protected]>
Signed-off-by: Miquel Raynal <[email protected]>
Link: https://lore.kernel.org/r/Message-Id: <[email protected]>
Signed-off-by: Mark Brown <[email protected]>

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