ASoC: cs35l56: Initialise a variable to silence possible static analysis error

read_poll_timeout() is a macro and val will be populated before use,
however some static analysis tools treat it as a function and warn of
uninitialised variable usage.

Signed-off-by: Simon Trimmer <simont@opensource.cirrus.com>
Signed-off-by: Richard Fitzgerald <rf@opensource.cirrus.com>
Link: https://lore.kernel.org/r/20231004144203.151775-1-rf@opensource.cirrus.com
Signed-off-by: Mark Brown <broonie@kernel.org>