68360serial: Plumb in rs_360_get_icount()

commit 3e517f4b1de4787ecff87a73a9865a0b1aa2b10b upstream.

Commit 0587102cf9f427c185bfdeb2cef41e13ee0264b1 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>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>