Staging: silicom: remove code requiring an old LINUX_VERSION_CODE

Remove all code and associated preprocessor logic dependent on an old
LINUX_VERSION_CODE since such code is dead for an in-kernel driver.

Signed-off-by: Chad Williamson <chad@dahc.us>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>