net,via-rhine: Fix tx_timeout handling

rhine_reset_task() misses to disable the tx scheduler upon reset,
this can lead to a crash if work is still scheduled while we're resetting
the tx queue.

Fixes:
[   93.591707] BUG: unable to handle kernel NULL pointer dereference at 0000004c
[   93.595514] IP: [<c119d10d>] rhine_napipoll+0x491/0x6

Signed-off-by: Richard Weinberger <richard@nod.at>
Signed-off-by: David S. Miller <davem@davemloft.net>