mfd: da9055: Do not cite as modular

This driver cannot be built as a module, so don't suggest otherwise
in Kconfig help.

Suggested-by: Lee Jones <lee@kernel.org>
Signed-off-by: Nick Alcock <nick.alcock@oracle.com>
Reviewed-by: Vincenzo Palazzo <vincenzopalazzodev@gmail.com>
Signed-off-by: Lee Jones <lee@kernel.org>