usb: hso: obey DMA rules in tiocmget

The serial state information must not be embedded into another
data structure, as this interferes with cache handling for DMA
on architectures without cache coherence..
That would result in data corruption on some architectures
Allocating it separately.

v2: fix syntax error

Signed-off-by: Oliver Neukum <oneukum@suse.com>
Signed-off-by: David S. Miller <davem@davemloft.net>