Bluetooth: Use memdup_user in le_auto_conn_write()

This patch does a small code simplification replacing the tipical
kmalloc-copy_from_user sequence by memdup_user() helper.

Cc: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Andre Guedes <andre.guedes@openbossa.org>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>