Staging: rtl8188eu: Fix warning of unchecked sscanf return value.

This patch fixes the warnings produced by checkpatch "unchecked sscanf
return value" in file ioctl_linux.c.

Signed-off-by: Elena Oat <oat.elena@gmail.com>
Signed-off-by: Peter P Waskiewicz Jr <peter.p.waskiewicz.jr@intel.com>