ata: ahci_st: Remove an unused field in struct st_ahci_drv_data

In "struct st_ahci_drv_data", the 'ahci' field is unused.
Remove it.

Found with cppcheck, unusedStructMember.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Damien Le Moal <dlemoal@kernel.org>