common/charge_state_v2: Use CONFIG_USB_PD_PREFER_MV to guard functions
common/charge_state_v2: Use CONFIG_USB_PD_PREFER_MV to guard functions

Guarding them for TEST_BUILD is not correct, as the functions may
still be needed for board tests.

Without this, make BOARD=kukui tests fails.

BRANCH=none
BUG=b:159571683
TEST=make BOARD=krane -j tests

Change-Id: Ib1be0ac55c15e610ff6cd13d746442c5b0e645ad
Signed-off-by: Nicolas Boichat <f3daba1cdc403799935674d3cd4b0e5bd1804232@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/2256630
Reviewed-by: Eric Yilun Lin <2f2733c7206ea2b76bd2db1905cc5e8a75841c48@chromium.org>

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