mb/sifive/hifive-unleashed: Use if (IS_ENABLED(...))

"if" is preferable over "#if", because it lets the compiler perform
syntax and type checks even if CONFIG_CONSOLE_SERIAL is disabled.

Change-Id: I45a763f2d854fbe9082795bc74de7a9d0fded3c9
Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Reviewed-on: https://review.coreboot.org/c/29336
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Philipp Hug <philipp@hug.cx>
Reviewed-by: Patrick Rudolph <siro@das-labor.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>

src/mainboard/sifive/hifive-unleashed/romstage.c

@@ -36,9 +36,8 @@ void main(void)
 	clock_init();
 
 	// re-initialize UART
-	#if (IS_ENABLED(CONFIG_CONSOLE_SERIAL))
+	if (IS_ENABLED(CONFIG_CONSOLE_SERIAL))
 		uart_init(CONFIG_UART_FOR_CONSOLE);
-	#endif
 
 	sdram_init();