net: thunderx: Fix format-truncation warning in bgx_acpi_match_id()

net: thunderx: Fix format-truncation warning in bgx_acpi_match_id()

The buffer bgx_sel used in snprintf() was too small to safely hold
the formatted string "BGX%d" for all valid bgx_id values. This caused
a -Wformat-truncation warning with `Werror` enabled during build.

Increase the buffer size from 5 to 7 and use `sizeof(bgx_sel)` in
snprintf() to ensure safety and suppress the warning.

Build warning:
  CC      drivers/net/ethernet/cavium/thunder/thunder_bgx.o
  drivers/net/ethernet/cavium/thunder/thunder_bgx.c: In function
‘bgx_acpi_match_id’:
  drivers/net/ethernet/cavium/thunder/thunder_bgx.c:1434:27: error: ‘%d’
directive output may be truncated writing between 1 and 3 bytes into a
region of size 2 [-Werror=format-truncation=]
    snprintf(bgx_sel, 5, "BGX%d", bgx->bgx_id);
                             ^~
  drivers/net/ethernet/cavium/thunder/thunder_bgx.c:1434:23: note:
directive argument in the range [0, 255]
    snprintf(bgx_sel, 5, "BGX%d", bgx->bgx_id);
                         ^~~~~~~
  drivers/net/ethernet/cavium/thunder/thunder_bgx.c:1434:2: note:
‘snprintf’ output between 5 and 7 bytes into a destination of size 5
    snprintf(bgx_sel, 5, "BGX%d", bgx->bgx_id);

compiler warning due to insufficient snprintf buffer size.

Signed-off-by: Alok Tiwari <alok.a.tiwari@oracle.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20250711140532.2463602-1-alok.a.tiwari@oracle.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>