net: Fix discuss discard typo

Replace discuss with discard, that is what happens with packet with
incorrect checksum. Fix the typo.

Fixes: 4b37fd14 ("Convert CONFIG_UDP_CHECKSUM to Kconfig")
Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Ramon Fried <rfried.dev@gmail.com>
Cc: Simon Glass <sjg@chromium.org>