staging: greybus: sdio: remove KERNEL_VERSION checks

No need to support older kernel versions in the Greybus SDIO driver, so
remove the checks as needed, we can now rely on all of the correct SDIO
core apis being present.

Signed-off-by: Rui Miguel Silva <rui.silva@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>