usb: musb: core: added missing const qualifier to musb_hdrc_platform_data::config

The musb_hdrc_platform_data::config was defined as a non-const pointer.
However some drivers (e.g. the ux500) set up this pointer to point to a
static structure, which is potentially dangerous. Since the musb core
uses the pointer in a read-only manner the const qualifier was added to
protect the content of the config.

Signed-off-by: Petr Kulhavy <petr@barix.com>
Acked-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: Bin Liu <b-liu@ti.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>