dell-wmi: sys_init_module: 'dell_wmi'->init suspiciously returned 21, it should follow 0/-E convention

wmi_install_notify_handler() returns an acpi_error,
but dell_wmi_init() needs return a -errno style error.
Signed-off-by: Len Brown <len.brown@intel.com>
Tested-by: Paul Rolland <rol@as2917.net>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>