mwifiex: sdio: don't check for NULL sdio_func

sdio_func is retrieved via container_of() and should never be NULL.
Checking for NULL just makes the logic more confusing than necessary.
Stop doing that.

Signed-off-by: Brian Norris <briannorris@chromium.org>
Tested-by: Xinming Hu <huxm@marvell.com>
Signed-off-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>