musb: otg timer cleanup

Minor cleanup of OTG timer handling:
    * unify decls for OTG time constants, in the core header
    * set up and use that timer in a more normal way
    * move to the driver struct, so it's usable outside core

And tighten use and setup of T(a_wait_bcon) so that if it's used,
it's always valid.  (If that timer expires, the A-device will
stop powering VBUS.  For non-OTG systems, that will be a surprise.)
No behavioral changes, other than more consistency when applying
that core HNP timeout.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>