pd: Validate size of discover identity received by DFP.
pd: Validate size of discover identity received by DFP.

Signed-off-by: Todd Broch <035f5b1f817cbb94019cec357b99778115602b63@chromium.org>

BRANCH=samus
BUG=chrome-os-partner:35859
TEST=manual, with CONFIG_CMD_USB_PD_PE and hoho in C1

> pe 1 dump
IDENT:
        [ID Header] 6c0018d1 :: AMA, VID:18d1
        [Cert Stat] 00000000
        [2] 50100001    [3] 1100000b
SVID[0]: ff01 MODES: [1] 00000485
SVID[1]: 18d1 MODES: [1] 00000001
MODE[1]: svid:ff01 caps:00000485

Now see only the 2 additional product type VDOs (product, AMA)
Bits still make sense.
  [2] 50100001 == 5010:Pid 0001:bcdDevice
  [3] 1100000b == 1:hw vers 1:fw version
                  b:vbus req, USB 2.0 billboard only

Change-Id: Ie8fb74fa55a25ee760009d5a2858a62b0f696c92
Reviewed-on: https://chromium-review.googlesource.com/243080
Trybot-Ready: Todd Broch <035f5b1f817cbb94019cec357b99778115602b63@chromium.org>
Tested-by: Todd Broch <035f5b1f817cbb94019cec357b99778115602b63@chromium.org>
Reviewed-by: Vincent Palatin <70a9964ec8fd10b0b08fcc8623c2add25ddf99ac@chromium.org>
Commit-Queue: Todd Broch <035f5b1f817cbb94019cec357b99778115602b63@chromium.org>

fourier49/BIZ_EC,fourier49/BIZ_EC,eatbyte/chromium-ec,eatbyte/chromium-ec,akappy7/ChromeOS_EC_LED_Diagnostics,fourier49/BZ_DEV_EC,md5555/ec,coreboot/chrome-ec,akappy7/ChromeOS_EC_LED_Diagnostics,coreboot/chrome-ec,akappy7/ChromeOS_EC_LED_Diagnostics,akappy7/ChromeOS_EC_LED_Diagnostics,fourier49/BZ_DEV_EC,md5555/ec,coreboot/chrome-ec,coreboot/chrome-ec,akappy7/ChromeOS_EC_LED_Diagnostics,fourier49/BIZ_EC,coreboot/chrome-ec,fourier49/BIZ_EC,fourier49/BZ_DEV_EC,eatbyte/chromium-ec,coreboot/chrome-ec,eatbyte/chromium-ec,md5555/ec,md5555/ec,fourier49/BZ_DEV_EC