staging: rtl8188eu: fix possible NULL dereference

dm_odm was being checked for NULL after dereferencing it. Lets check for
NULL first before derefenrencing it.

Signed-off-by: Sudip Mukherjee <sudip@vectorindia.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>