spi: spi-loopback-test: Add module param for iteration length

SPI test framework is designed to run each test case for
a list of lengths.
Introduce a module parameter to limit the iterations
to a single value among the list of lengths.

Signed-off-by: Rohit Ner <rohitner@google.com>
Link: https://lore.kernel.org/r/20230412202009.3750955-2-rohitner@google.com


Signed-off-by: Mark Brown <broonie@kernel.org>