platform/x86: dell-smbios-wmi: Fix oops on rmmod dell_smbios

In the Linux kernel, the following vulnerability has been resolved:

platform/x86: dell-smbios-wmi: Fix oops on rmmod dell_smbios

init_dell_smbios_wmi() only registers the dell_smbios_wmi_driver on systems

where the Dell WMI interface is supported. While exit_dell_smbios_wmi()

unregisters it unconditionally, this leads to the following oops:

[ 175.722921] ------------[ cut here ]------------

[ 175.722925] Unexpected driver unregister!

[ 175.722939] WARNING: CPU: 1 PID: 3630 at drivers/base/driver.c:194 driver_unregister+0x38/0x40

...

[ 175.723089] Call Trace:

[ 175.723094] cleanup_module+0x5/0xedd [dell_smbios]

...

[ 175.723148] ---[ end trace 064c34e1ad49509d ]---

Make the unregister happen on the same condition the register happens

to fix this.