USB: serial: fix return value for unsupported ioctls

USB: serial: fix return value for unsupported ioctls

Drivers should return -ENOTTY ("Inappropriate I/O control operation")
when an ioctl isn't supported, while -EINVAL is used for invalid
arguments.

Fix up the TIOCMGET, TIOCMSET and TIOCGICOUNT helpers which returned
-EINVAL when a USB serial driver did not implement the corresponding
methods.

Note that the TIOCMGET and TIOCMSET helpers predate git and do not get a
corresponding Fixes tag below.

Fixes: d281da7ff6f7 ("tty: Make tiocgicount a handler")
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>