Remove device reference from struct ltc2991_state since it is used only
inside the init function.

Pass the struct device as parameter to the init function instead.
Signed-off-by: Antoniu Miclaus <antoniu.miclaus@analog.com>
Link: https://lore.kernel.org/r/20231031095647.48376-1-antoniu.miclaus@analog.comSigned-off-by: Guenter Roeck <linux@roeck-us.net>