can: flexcan: move rx_offload_add() from flexcan_probe() to flexcan_open()

rx offload depends on number of message buffers, which in turn depends
on messgae buffer size. with the upcoming LX2160A SOC the message buffer
size can be configured to 72 bytes if it were to be used in CAN FD mode.

The current mode in which the flexcan is being operated is known at the
time of flexcan_open() but not at the time of flexcan_probe().

Therefore, move the rx_offload_add() from flexcan_probe() to
flexcan_open(). correspondingly, move rx_offload_delete() from
flexcan_remove() to flexcan_close().

Signed-off-by: Pankaj Bansal <pankaj.bansal@nxp.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>