axi: fix definition of axi_sandbox_get_emul()

axi: fix definition of axi_sandbox_get_emul()

Compiling with gcc 13 results in an error:

    drivers/axi/axi-emul-uclass.c:16:5: warning: conflicting types for
    ‘axi_sandbox_get_emul’ due to enum/integer mismatch; have
    ‘int(struct udevice *, ulong,  enum axi_size_t,  struct udevice **)’
    {aka ‘int(struct udevice *, long unsigned int,  enum axi_size_t,
    struct udevice **)’} [-Wenum-int-mismatch]
       16 | int axi_sandbox_get_emul(struct udevice *bus, ulong address,
          |     ^~~~~~~~~~~~~~~~~~~~
    In file included from drivers/axi/axi-emul-uclass.c:14:
    ./arch/sandbox/include/asm/axi.h:48:5: note: previous declaration of
    ‘axi_sandbox_get_emul’ with type ‘int(struct udevice *, ulong,  uint,
    struct udevice **)’ {aka ‘int(struct udevice *, long unsigned int,
    unsigned int,  struct udevice **)’}
       48 | int axi_sandbox_get_emul(struct udevice *bus, ulong address, uint length,
          |     ^~~~~~~~~~~~~~~~~~~~

Adjust the header definition to match the implementation.
Define the size parameter as constant.

Fixes: 9a8bcabd8ada ("axi: Add AXI sandbox driver and simple emulator")
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Simon Glass <sjg@chromium.org>