cmd_bootm.c: Fix problem with '#if (CONFIG_CMD_USB)'

A recent patch used '#if (CONFIG_CMD_USB)' instead of
'#if defined(CONFIG_CMD_USB)'. This patch fixes this problem and makes
common/bootm.c compile again.

Signed-off-by: Stefan Roese <sr@denx.de>
Acked-by: Markus Klotzbuecher <mk@denx.de>