drivers: spi: remove '&' when assigning `init_fn`

Remove address-of operator ('&') when assigning `init_fn`
function pointer in `DEVICE_DT_INST_DEFINE` macro.

This change aims to maintain consistency among the drivers in
`drivers/spi`, ensuring that all function pointer assignments
follow the same pattern.

Signed-off-by: Pisit Sawangvonganan <pisit@ndrsolution.com>