pata_hpt3x2n: fix writing to wrong register in hpt3x2n_bmdma_stop()

The driver's bmdma_stop() method writes to the wrong PCI config register
(0x52 intead of 0x54) when trying to clear the state machine on secondary
channel -- "luckily", the write falls on a read-only part of the primary
channel MISC. control 3 register, so no collateral damage is done...

Alan Cox fixed the HPT37x driver in commit 6929da44 ("[PATCH] hpt37x:
Two important bug fixes") but forgot to check the HPT3x2N driver which has
the same bug. :-/

Signed-off-by: Sergey Shtylyov <s.shtylyov@omp.ru>
Signed-off-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>