Revert "USB: serial: fix hang when opening port"

This reverts commit eba0e3c3.

When merged together (usb-linus and usb-next), this fix isn't needed and
causes a build error.  Revert the commit to solve the build issue.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: Ming Lei <tom.leiming@gmail.com>
Cc: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>