Bluetooth: Fix wrong memcpy size on LE start encryption

This patch fixes wrong memcpy size when copying rand value to
HCI_OP_LE_START_ENC command.
The compiler pretends that the array parameter was declared as a pointer
and sizeof reports the size of the pointer. [1]

[1] http://www.c-faq.com/aryptr/aryparmsize.html

Signed-off-by: Anderson Briglia <anderson.briglia@openbossa.org>
Signed-off-by: Anderson Lizardo <anderson.lizardo@openbossa.org>
Signed-off-by: Gustavo F. Padovan <padovan@profusion.mobi>