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.fr


Signed-off-by: Lee Jones <lee@kernel.org>