iwl3945: fix length of dma buffers

commit bdb084b2
Author: Stanislaw Gruszka <sgruszka@redhat.com>
Date:   Wed Feb 13 15:49:08 2013 +0100

    iwlegacy: more checks for dma mapping errors

broke il3945_tx_skb() dma buffer length settings, what results on
firmware errors like showed below and make 3945 device non usable.

iwl3945 0000:02:00.0: Microcode SW error detected. Restarting 0x82000008.
iwl3945 0000:02:00.0: Loaded firmware version: 15.32.2.9
iwl3945 0000:02:00.0: Start IWL Error Log Dump:
iwl3945 0000:02:00.0: Status: 0x000202E4, count: 1
iwl3945 0000:02:00.0: Desc 	Time	   asrtPC blink2 ilink1  nmiPC   Line
iwl3945 0000:02:00.0: SYSASSERT     (0x5) 0000208934 0x008B6 0x0035E 0x00320 0x00000 267
iwl3945 0000:02:00.0: Error Reply type 0x00000001 cmd

Reported-by: Zdenek Kabelac <zkabelac@redhat.com>
Reported-by: Krzysztof Kolasa <kkolasa@winsoft.pl>
Reported-by: Pedro Francisco <pedrogfrancisco@gmail.com>
Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>