wireless: at76c50x: signedness bug in at76_dfu_get_state()

This return holds the number of bytes transfered (1 byte) or a negative
error code.  The type should be int instead of u8.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Pavel Roskin <proski@gnu.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>