MUSB: Add sanity check for maximum number of endpoints

There is no check if platform code passes in more endpoints (num_eps)
than the maximum number of enpoints (MUSB_C_NUM_EPS.)  The result is
that allocate_instance() happily writes past the end of 'struct musb'
corrupting memory.

This patch adds a BUG() if the platform code requests more than the max.

Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>
Acked-by: Felipe Balbi <felipe.balbi@nokia.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>