net: liquidio: fix clang-specific W=1 build warnings

When compiling with clang-18 and W=1, I've noticed the following
warnings:

drivers/net/ethernet/cavium/liquidio/cn23xx_pf_device.c:1493:16: warning: cast
from 'void (*)(struct octeon_device *, struct octeon_mbox_cmd *, void *)' to
'octeon_mbox_callback_t' (aka 'void (*)(void *, void *, void *)') converts to
incompatible function type [-Wcast-function-type-strict]
 1493 |         mbox_cmd.fn = (octeon_mbox_callback_t)cn23xx_get_vf_stats_callback;
      |                       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

and:

drivers/net/ethernet/cavium/liquidio/cn23xx_vf_device.c:432:16: warning: cast
from 'void (*)(struct octeon_device *, struct octeon_mbox_cmd *, void *)' to
'octeon_mbox_callback_t' (aka 'void (*)(void *, void *, void *)') converts to
incompatible function type [-Wcast-function-type-strict]
  432 |         mbox_cmd.fn = (octeon_mbox_callback_t)octeon_pfvf_hs_callback;
      |                       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Fix both of the above by adjusting 'octeon_mbox_callback_t' to match actual
callback definitions (at the cost of adding an extra forward declaration).

Signed-off-by: Dmitry Antipov <dmantipov@yandex.ru>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://lore.kernel.org/r/20240111162432.124014-1-dmantipov@yandex.ru
Signed-off-by: Jakub Kicinski <kuba@kernel.org>