musb_host: refactor URB giveback

As musb_advance_schedule() is now the only remaning
caller of musb_giveback() (and the only valid context
of such call), just fold the latter into the former
and then rename __musb_giveback() into musb_giveback().

This is a net minor shrink.

Signed-off-by: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>