xen/serial: scif: Rework how the parameters are found

xen/serial: scif: Rework how the parameters are found

clang 11 will throw the following error while build Xen:

scif-uart.c:333:33: error: cast to smaller integer type 'enum port_types' from 'const void *' [-Werror,-Wvoid-pointer-to-enum-cast]
    uart->params = &port_params[(enum port_types)match->data];
                                ^~~~~~~~~~~~~~~~~~~~~~~~~~~~

The error can be prevented by directly storing a pointer to the port
parameters rather than the a cast of the port type.

Signed-off-by: Julien Grall <jgrall@amazon.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>