tty/powerpc: early udbg consoles can't be modules

Fixes these build errors:

ERROR: ".udbg_printf" [drivers/tty/ehv_bytechan.ko] undefined!
ERROR: ".register_early_udbg_console" [drivers/tty/ehv_bytechan.ko] undefined!
ERROR: "udbg_putc" [drivers/tty/ehv_bytechan.ko] undefined!

Cc: Timur Tabi <timur@freescale.com>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/Kconfig

@@ -365,7 +365,7 @@ config PPC_EPAPR_HV_BYTECHAN
 
 config PPC_EARLY_DEBUG_EHV_BC
 	bool "Early console (udbg) support for ePAPR hypervisors"
-	depends on PPC_EPAPR_HV_BYTECHAN
+	depends on PPC_EPAPR_HV_BYTECHAN=y
 	help
 	  Select this option to enable early console (a.k.a. "udbg") support
 	  via an ePAPR byte channel.  You also need to choose the byte channel