wifi: virt_wifi: don't use strlen() in const context

wifi: virt_wifi: don't use strlen() in const context

Looks like not all compilers allow strlen(constant) as
a constant, so don't do that. Instead, revert back to
defining the length as the first submission had it.

Fixes: b5d14b0c6716 ("wifi: virt_wifi: avoid reporting connection success with wrong SSID")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202407090934.NnR1TUbW-lkp@intel.com/
Closes: https://lore.kernel.org/oe-kbuild-all/202407090944.mpwLHGt9-lkp@intel.com/
Signed-off-by: Johannes Berg <johannes.berg@intel.com>