dynamic_debug: provide dynamic_hex_dump stub

The ionic driver started using dymamic_hex_dump(), but
that is not always defined:

drivers/net/ethernet/pensando/ionic/ionic_main.c:229:2: error: implicit declaration of function 'dynamic_hex_dump' [-Werror,-Wimplicit-function-declaration]

Add a dummy implementation to use when CONFIG_DYNAMIC_DEBUG
is disabled, printing nothing.

Fixes: 938962d5 ("ionic: Add adminq action")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Shannon Nelson <snelson@pensando.io>
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>