Bluetooth: Increase HCI_LE_rand delay for MyNewt

1 tick (10 ms on most current boards) doesn't seem to be enough to
completely eliminate the HCI_LE_Rand bug. Increase the delay to 20 ms
(i.e. 2 ticks).

Change-Id: Ibe87d5e4b2ec43b2f96afb2c4e0ac726ffe99bd6
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>