hw/ide/mmio: Use CamelCase for MMIO_IDE state name

Following docs/devel/style.rst guidelines, rename MMIOIDEState
as IdeMmioState.

Having the structure name and its typedef named equally,
we can manually convert from the old DECLARE_INSTANCE_CHECKER()
macro to the more recent OBJECT_DECLARE_SIMPLE_TYPE().

Note, due to that name mismatch, this macro wasn't automatically
converted during commit 8063396bf3 ("Use OBJECT_DECLARE_SIMPLE_TYPE
when possible").

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20230220091358.17038-3-philmd@linaro.org>