mwifiex: claim sdio bus while downloading the firmware

Our 8887 A2 chip can have separate firmware images for
bluetooth and WLAN. We observed an issue during parallel
downloading of these images by btmrvl and mwifiex drivers.

This patch claims sdio bus before starting of the firmware
download in mwifiex and releases it after completion to
fix the problem.

Signed-off-by: Aniket Nagarnaik <aniketn@marvell.com>
Signed-off-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>