hp-wmi: handle rfkill_register() failure

Compilation of the HP WMI hotkeys code results in the following:

  CC [M]  drivers/platform/x86/hp-wmi.o
drivers/platform/x86/hp-wmi.c: In function hp_wmi_bios_setup:
drivers/platform/x86/hp-wmi.c:431: warning: ignoring return value of rfkill_register,
	 declared with attribute warn_unused_result
drivers/platform/x86/hp-wmi.c:441: warning: ignoring return value of rfkill_register,
	 declared with attribute warn_unused_result
drivers/platform/x86/hp-wmi.c:450: warning: ignoring return value of rfkill_register,
	 declared with attribute warn_unused_result

Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Cc: Matthew Garrett <mjg59@srcf.ucam.org>
Cc: Len Brown <lenb@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>