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>

drivers/ata/ahci_st.c

@@ -30,7 +30,6 @@
 #define ST_AHCI_OOBR_CIMAX_SHIFT	0
 
 struct st_ahci_drv_data {
-	struct platform_device *ahci;
 	struct reset_control *pwr;
 	struct reset_control *sw_rst;
 	struct reset_control *pwr_rst;