68360serial: Plumb in rs_360_get_icount()

Commit 0587102c replaced a direct
implementation of SIOCGICOUNT with an implementation of
tty_operations::get_icount, but it did not actually set
rs_360_ops.get_icount.

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Cc: stable@kernel.org [2.6.37]
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>