net/smc: set rx_off for SMCR explicitly

SMC tries to make use of SMCD first. If a problem shows up,
it tries to switch to SMCR. If the SMCD initializing problem shows
up after the SMCD connection has already been initialized, field
rx_off keeps the wrong SMCD value for SMCR, which results in corrupted
data at the receiver.
This patch adds an explicit (re-)setting of field rx_off to zero if the
connection uses SMCR.

Fixes: be244f28

 ("net/smc: add SMC-D support in data transfer")
Reviewed-by: Karsten Graul <kgraul@linux.ibm.com>
Signed-off-by: Ursula Braun <ubraun@linux.ibm.com>
Signed-off-by: Karsten Graul <kgraul@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>