ath10k: sdio: fix compile warning

As suggested by Arnd Bergmann, replace
"while (time_before_...) {}"
with
"do {} while (time_before_...)"

This fixes the following warnings detected by gcc 4.1.2:

drivers/net/wireless/ath/ath10k/sdio.c: In function
‘ath10k_sdio_mbox_rxmsg_pending_handler’:
drivers/net/wireless/ath/ath10k/sdio.c:676: warning: ‘ret’ may be used uninitialized in this function

...

drivers/net/wireless/ath/ath10k/sdio.c: In function
‘ath10k_sdio_irq_handler’:
drivers/net/wireless/ath/ath10k/sdio.c:1331: warning: ‘ret’ may be used uninitialized in this function

Signed-off-by: Erik Stromdahl <erik.stromdahl@gmail.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>