[media] mxl111sf: Don't use dynamic static allocation
Dynamic static allocation is evil, as Kernel stack is too low, and
compilation complains about it on some archs:
drivers/media/usb/dvb-usb-v2/mxl111sf.c:74:1: warning: 'mxl111sf_ctrl_msg' uses dynamic stack allocation [enabled by default]
Instead, let's enforce a limit for the buffer to be the max size of
a control URB payload data (64 bytes).