bnxt_en: hide CONFIG_DETECT_HUNG_TASK specific code

bnxt_en: hide CONFIG_DETECT_HUNG_TASK specific code

The CONFIG_DEFAULT_HUNG_TASK_TIMEOUT setting is only available when the
hung task detection is enabled, otherwise the code now produces a build
failure:

drivers/net/ethernet/broadcom/bnxt/bnxt.c:10188:21: error: use of undeclared identifier 'CONFIG_DEFAULT_HUNG_TASK_TIMEOUT'
 10188 |             max_tmo_secs > CONFIG_DEFAULT_HUNG_TASK_TIMEOUT) {

Enclose this warning logic in an #ifdef to ensure this builds.

Fixes: 0fcad44a86bd ("bnxt_en: Change FW message timeout warning")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Michael Chan <michael.chan@broadcom.com>
Link: https://patch.msgid.link/20250423162827.2189658-1-arnd@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>