Bluetooth: hci_ecc: Verify LE Generate DHKey command parameters

Reply with Invalid Parameters Command Status if incomplete command
was received.

 HCI Command: LE Generate DHKey (0x08|0x0026) plen 60
        invalid packet size
        f6 0f 79 a0 3c 4f 3e 90 40 66 6b 1d 93 64 af ed  ..y.<O>.@fk..d..
        03 ec b4 5f 29 cf 6d 26 42 02 61 b9 29 9a bf 0b  ..._).m&B.a.)...
        5e da 86 8b 3a ef aa e8 13 3d 87 de a7 dc 36 57  ^...:....=....6W
        25 89 56 43 e8 2a 73 56 62 60 fa 81              %.VC.*sVb`..
> HCI Event: Command Status (0x0f) plen 4
      LE Generate DHKey (0x08|0x0026) ncmd 1
        Status: Invalid HCI Command Parameters (0x12)

Change-Id: I9c4e798096023f0807e8d353ac3318e3d13dc6f7
Signed-off-by: Szymon Janc <szymon.janc@codecoup.pl>