staging: most: i2c: remove redundant is_open

The variable is_open is checked only in the work function
pending_rx_work() that is only active between the calls
configure_channel() and poison_channel().

Signed-off-by: Andrey Shvetsov <andrey.shvetsov@k2l.de>
Signed-off-by: Christian Gromm <christian.gromm@microchip.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>