libertas: avoid -Wempty-body warning

Building without mesh supports shows a couple of warnings with
'make W=1':

drivers/net/wireless/marvell/libertas/main.c: In function 'lbs_start_card':
drivers/net/wireless/marvell/libertas/main.c:1068:37: error: suggest braces around empty body in an 'if' statement [-Werror=empty-body]
 1068 |                 lbs_start_mesh(priv);

Change the macros to use the usual "do { } while (0)" instead to shut up
the warnings and make the code a litte more robust.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20210322104343.948660-4-arnd@kernel.org