tests: samples: Exclude nrf52840_pca10056 from wpanusb sample

With usb_device support added to nrf52840_pca10056 board, sanity
check script compiles the sample net/wpanusb and it fails with
undefined reference for symbol cc2520_configure_gpios.

First of all cc2520 shouldn't be compiled at all for nrf52840
since it has its own ieee802154 radio. The prj.conf under the
sample samples/net/wpanusb simply hard codes the config
CONFIG_IEEE802154_CC2520=y and is causing the ieee80211_cc2520.c
file to be compiled. If it's removed from the configuration file
we get other errors. So it is not straight forward.

For now, as a workaround, exclude the nrf52840_pca10056 platform
until there's a proper fix.

Signed-off-by: Sundar Subramaniyan <sundar.subramaniyan@gmail.com>