rsi: fix a dereference on adapter before it has been null checked

The assignment of dev is dereferencing adapter before adapter has
been null checked, potentially leading to a null pointer dereference.
Fix this by simply moving the assignment of dev to a later point
after the sanity null check of adapter.

Detected by CoverityScan CID#1398383 ("Dereference before null check")

Fixes: dad0d04f ("rsi: Add RS9113 wireless driver")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>