USB: serial gadget: simplify endpoint handling

Switch serial gadget away from a *very* old idiom:  just remember
the endpoints we'll be using, instead of looking them up by name
each time.  This is a net code and data (globals) shrink.

Also fix a small memory leak in the rmmod path, by working the
same as the disconnect code.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Cc: Al Borchers <alborchers@steinerpoint.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>