staging/nvec:: avoid Wempty-body warning

This driver has a few disabled diagnostics, which can probably
just get removed, or might still be helpful:

drivers/staging/nvec/nvec_ps2.c: In function 'nvec_ps2_notifier':
drivers/staging/nvec/nvec_ps2.c:94:77: error: suggest braces around empty body in an 'if' statement [-Werror=empty-body]
   94 |                         NVEC_PHD("unhandled mouse event: ", msg, msg[1] + 2);

Changing the empty macro to the usual 'do {} while (0)' at least
shuts up the compiler warnings.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20210322103545.704121-2-arnd@kernel.org


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>