mwifiex: code rearrangement in pcie.c and sdio.c

Next patch in this series is going to use mwifiex_read_reg() in remove
handlers. The changes here are prerequisites to avoid forward
declarations.

Signed-off-by: Xinming Hu <huxm@marvell.com>
Signed-off-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>