ata: ahci_st: Fix compilation warning

If CONFIG_OF is disabled and the ahci_st driver is builtin (or
CONFIG_MODULES is disabled), then using the macro of_match_ptr()
results in the st_ahci_match variable being unused, which generates a
compilation warning and a compilation error if CONFIG_WERROR is enabled.

Fix this by directly assigning st_ahci_match to .of_match_table in the
st_ahci_driver platform driver definition.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>