usb: Prevent using reserved registers on DM36x usb

The musb driver defines and uses MUSB_CSR0_H_DIS_PING, however this
bit is reserved on the DM36x. Thus this patch ensures that the
reserved bit is not accesssed.

It has been observed that some USB devices will fail to enumerate
with errors such as 'error in inquiry' without this patch.

See http://www.ti.com/litv/pdf/sprufh9a

 for details.

Cc: Marek Vasut <marex@denx.de>
Cc: Tom Rini <trini@ti.com>
Signed-off-by: Andrew Murray <amurray@embedded-bits.co.uk>
Acked-by: Marek Vasut <marex@denx.de>