mwifiex: fix racing condition when downloading firmware

The action 'check for winner' and 'download firmware' should be an
atomic action. This is true for btmrvl driver but not mwmfiex, which
cause firmware download to fail when the following senerio happens:

1) mwifiex check winner status: true
2) btmrvl check winner status: true, and start downloading firmware
3) mwfieix tries to download firmware, but failed because btmrvl is
already downloading.

This won't happen if 1) and 3) is an atomic action. This patch adds
sdio_claim/release_host call around those two actions to make sure it's
atomic.

Signed-off-by: Wei-Ning Huang <wnhuang@chromium.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>