mwifiex: fix link error against sdio

Calling sdio_claim_host() from the interface independent part of
the mwifiex driver is not only a layering violation, but also causes
a link error if MMC support is disabled, or if CONFIG_MMC=m
and CONFIG_MWIFIEX=y:

drivers/net/built-in.o: In function `mwifiex_fw_dpc':
:(.text+0xff138): undefined reference to `sdio_claim_host'
:(.text+0xff158): undefined reference to `sdio_release_host'

The right way to do this is to have the sdio specific code in the
sdio driver front-end, and we already have a callback pointer that
we can use for this after exporting the generic fw download
function from the core driver.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: 65c71efe ("mwifiex: fix racing condition when downloading firmware")
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>