zephyr: shim: add default to PLATFORM_EC_USB_PID
zephyr: shim: add default to PLATFORM_EC_USB_PID

All Kconfig symbols are added to the binary with the
GEN_ABSOLUTE_SYM_KCONFIG. This is a problem with hex symbols with a
default empty value, as they end up defined to "0x", causing an obscure
build failure such as:

FAILED: zephyr/CMakeFiles/zephyr.dir/misc/generated/configs.c.obj
/tmp/cc5ZD1EK.s: Assembler messages:
/tmp/cc5ZD1EK.s:1345: Error: missing expression

Setting a default for the option so that that would not break the build.

BUG=none
BRANCH=none
TEST=drop the config for a board, build it, no assembler errors

Signed-off-by: Fabio Baltieri <1c55e39c05d9ab42261288cf330256b2af6ba179@google.com>
Change-Id: Ib886cafdccf7d7da21b0fe90b03aa40cc55c2715
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/3801901
Reviewed-by: Madhu P <91daf3dcb52497aaee2bfb2e83426acf7b0e812c@google.com>
Reviewed-by: Scott Collyer <7d40b3eb9e17bf55d972074a5e77a7ea4159f9dd@chromium.org>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec