usb: Tidy up the usb_start flag

This should be declared in a header file so that type-checking works
correctly.

Add a single declaration to usb.h and remove the others.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Marek Vasut <marex@denx.de>
Reviewed-by: Patrick Delaunay <patrick.delaunay@foss.st.com>