USB: musb: kill compile warning for Blackfin systems

The Blackfin version of musb_read_target_reg_base() returns a u16 when the
common code expects a (void __iomem *), so update the Blackfin function to
return the right value.  This fixes the compile warning:

drivers/usb/musb/musb_core.c: In function 'musb_core_init':
drivers/usb/musb/musb_core.c:1448: warning: assignment makes pointer from
	integer without a cast
Signed-off-by: Bryan Wu <cooloney@kernel.org>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Anand Gadiyar <gadiyar@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/usb/musb/musb_regs.h

@@ -465,9 +465,9 @@ static inline u16 musb_read_hwvers(void __iomem *mbase)
 	return 0;
 }
 
-static inline u16 musb_read_target_reg_base(u8 i, void __iomem *mbase)
+static inline void __iomem *musb_read_target_reg_base(u8 i, void __iomem *mbase)
 {
-	return 0;
+	return NULL;
 }
 
 static inline void musb_write_rxfunaddr(void __iomem *ep_target_regs,