usb: musb: disable double buffering when it's broken

We know that blackfin doesn't support double
buffering feature as of today. So we add a
flag set by musb_platform_init() to forcefully
disable that feature.

Such flag is created and marked as deprecated
to force us to find a solution for the missing
double buffering support on blackfin.

Signed-off-by: Felipe Balbi <balbi@ti.com>