The previous memcpy implementation relied on the size advertized by the
device. There were no guarantees that buf was big enough.

Some gymnastic is also required with the +2/-2 to take into account
the first 2 bytes of the returned buffer where the total returned
length is supplied by the device.
Signed-off-by: NBenjamin Tissoires <benjamin.tissoires@gmail.com>
Signed-off-by: NJiri Kosina <jkosina@suse.cz>