altera-stapl: check for a null key before strcasecmp'ing it

Currently the null check on key is occurring after the strcasecmp on
the key, hence there is a potential null pointer dereference on key.
Fix this by checking if key is null first. Also replace the == 0
check on strcasecmp with just the ! operator.

Detected by CoverityScan, CID#1248787 ("Dereference before null check")

Fixes: fa766c9b ("[media] Altera FPGA firmware download module")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>