mwifiex: set adapter->dev before starting to use mwifiex_dbg()

The mwifiex_dbg() log handler utilizes the struct device in
adapter->dev. Without it, it decides not to print anything.

As of commit 2e02b581 ("mwifiex: Allow mwifiex early access to device
structure"), we started assigning that pointer only after we finished
mwifiex_register() -- this effectively neuters any mwifiex_dbg() logging
done before this point.

Let's move the device assignment into mwifiex_register().

Fixes: 2e02b581 ("mwifiex: Allow mwifiex early access to device structure")
Cc: Rajat Jain <rajatja@google.com>
Signed-off-by: Brian Norris <briannorris@chromium.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>