libertas: fix suspend and resume for SDIO connected cards

Prior to commit 573185cc ("mmc: core: Invoke sdio func driver's PM
callbacks from the sdio bus"), the MMC core used to call into the power
management functions of SDIO clients itself and removed the card if the
return code was non-zero. IOW, the mmc handled errors gracefully and didn't
upchain them to the pm core.

Since this change, the mmc core relies on generic power management
functions which treat all errors as a reason to cancel the suspend
immediately. This causes suspend attempts to fail when the libertas
driver is loaded.

To fix this, power down the card explicitly in if_sdio_suspend() when we
know we're about to lose power and return success. Also set a flag in these
cases, and power up the card again in if_sdio_resume().

Fixes: 573185cc

 ("mmc: core: Invoke sdio func driver's PM callbacks from the sdio bus")
Cc: <stable@vger.kernel.org>
Signed-off-by: Daniel Mack <daniel@zonque.org>
Reviewed-by: Chris Ball <chris@printf.net>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>