USB: move the definition of USB_MAXCHILDREN

The USB_MAXCHILDREN symbol is used in include/uapi/linux/usb/ch11.h, a
user-mode header, even though it is defined in include/linux/usb.h,
which is kernel-only.  This causes compile-time errors when user
programs try to #include linux/usb/ch11.h.

This patch fixes the problem by moving the definition of USB_MAXCHILDREN
into ch11.h.  It also gets rid of unneeded parentheses.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>