staging: wlan-ng fix buffer overflow in firmware handling

We test for an END marker in the element beyond the current one, this
effectively limits the size of the array to be HFA384x_PDA_LEN_MAX/2 - 1
not HFA384x_PDR_END_OF_PDA/2. This patch fixes a possible buffer
overflow in case there was no END marker.

Signed-off-by: Tillmann Heidsieck <theidsieck@leenox.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>