platform/x86: oxpec: Move fan speed read to separate function

platform/x86: oxpec: Move fan speed read to separate function

While not necessary for fixing the ABI hwmon issue, fan speed will be
the only remaining value without a function. Therefore, finish the
refactor by moving it to a separate function.

Reviewed-by: Derek J. Clark <derekjohn.clark@gmail.com>
Reviewed-by: Thomas Weißschuh <linux@weissschuh.net>
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Antheas Kapenekakis <lkml@antheas.dev>
Link: https://lore.kernel.org/r/20250425111821.88746-11-lkml@antheas.dev
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>