Gcc 15 has changed how unions are initialized and now this driver gets an
uninitialized variable warning.

Reported as Gcc bug. https://gcc.gnu.org/bugzilla/show_bug.cgi?id=118403

Simple fix is to change to empty initializer.

2377/3224] Compiling C object
drivers/net/thunderx/base/libnicvf_base.a.p/nicvf_mbox.c.o
In function ‘nicvf_mbox_send_msg_to_pf_raw’,
    inlined from ‘nicvf_mbox_send_async_msg_to_pf’ at
../drivers/net/thunderx/base/nicvf_mbox.c:70:2,
    inlined from ‘nicvf_mbox_cfg_done’ at
../drivers/net/thunderx/base/nicvf_mbox.c:468:2:
../drivers/net/thunderx/base/nicvf_mbox.c:59:17: warning: ‘mbx’ is used
uninitialized [-Wuninitialized]
   59 |                 nicvf_reg_write(nic, mbx_addr, *mbx_data);
      |                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../drivers/net/thunderx/base/nicvf_mbox.c: In function ‘nicvf_mbox_cfg_done’:
../drivers/net/thunderx/base/nicvf_mbox.c:465:24: note: ‘mbx’ declared here
  465 |         struct nic_mbx mbx = { .msg = { 0 } };
      |                        ^~~
In function ‘nicvf_mbox_send_msg_to_pf_raw’,
    inlined from ‘nicvf_mbox_send_async_msg_to_pf’ at
../drivers/net/thunderx/base/nicvf_mbox.c:70:2,
    inlined from ‘nicvf_mbox_link_change’ at
../drivers/net/thunderx/base/nicvf_mbox.c:477:2:
../drivers/net/thunderx/base/nicvf_mbox.c:59:17: warning: ‘mbx’ is used
uninitialized [-Wuninitialized]
   59 |                 nicvf_reg_write(nic, mbx_addr, *mbx_data);
      |                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../drivers/net/thunderx/base/nicvf_mbox.c: In function
‘nicvf_mbox_link_change’:
../drivers/net/thunderx/base/nicvf_mbox.c:474:24: note: ‘mbx’ declared here
  474 |         struct nic_mbx mbx = { .msg = { 0 } };