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: NStefan Roese <sr@denx.de>
Acked-by: NMarkus Klotzbuecher <mk@denx.de>

common/cmd_bootm.c

@@ -36,7 +36,7 @@
 #include <lmb.h>
 #include <asm/byteorder.h>
 
-#if (CONFIG_CMD_USB)
+#if defined(CONFIG_CMD_USB)
 #include <usb.h>
 #endif
 
@@ -217,7 +217,7 @@ int do_bootm (cmd_tbl_t *cmdtp, int flag, int argc, char *argv[])
 	 */
 	iflag = disable_interrupts();
 
-#if (CONFIG_CMD_USB)
+#if defined(CONFIG_CMD_USB)
 	/*
 	 * turn off USB to prevent the host controller from writing to the
 	 * SDRAM while Linux is booting. This could happen (at least for OHCI