ata: ahci_st: Fix compilation warning

Remove the unused variable dev in st_ahci_probe() to avoid compilation
warning and build failures where CONFIG_WERROR is enabled.

Fixes: 3f74cd04

 ("ata: libahci_platform: Parse ports-implemented property in resources getter")
Signed-off-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>