s390/qeth: restart pending READ cmd from callback

The completion of a pending READ cmd is processed via
qeth_issue_next_read_cb(). Let this callback also start the next READ
cmd, instead of hardcoding that step into the IRQ handler.

While at it remove the check of the channel state,
__qeth_issue_next_read() already does this.

Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>