mfd: ssbi: Remove unused field 'slave' from 'struct ssbi'

In 'struct ssbi, the 'slave' field is unused. Remove it.

Found with cppcheck, unusedStructMember.
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Link: https://lore.kernel.org/r/8a76de25cefb533d94dfe35062bbd9a8e72f4bb9.1713971415.git.christophe.jaillet@wanadoo.frSigned-off-by: Lee Jones <lee@kernel.org>

drivers/mfd/ssbi.c

@@ -64,7 +64,6 @@ enum ssbi_controller_type {
 };
 
 struct ssbi {
-	struct device		*slave;
 	void __iomem		*base;
 	spinlock_t		lock;
 	enum ssbi_controller_type controller_type;