mwl8787: fix bad pointer arithmetic

cast takes precedence over minus.

Signed-off-by: Bob Copeland <me@bobcopeland.com>
mwl8787: send correct packet length to sdio functions

It needs to be the rounded-up block size, not number of blocks,
and should include interface header length.

Signed-off-by: Bob Copeland <me@bobcopeland.com>