ata: sata_highbank: remove superfluous cast

hpriv->plat_data is 'void *' so there is no need to cast it to
'struct ecx_plat_data *'.

Cc: Mark Langsdorf <mark.langsdorf@calxeda.com>
Cc: Rob Herring <robh@kernel.org>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Signed-off-by: Tejun Heo <tj@kernel.org>