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>