USB: serial: remove hupping check from tiocmiwait

Since commit 31ca020b ("TTY: wake up processes last at hangup") there no
longer any need to check the hupping flag in the generic tiocmiwait
implementation, so remove it.

Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>