msm: ramdump: fix duplicated sysfs ramdump

subsystem creates sysfs for ramdump. In some cases, it tries to create
same name sysfs which is created previously in other subsystem. To fix
duplicated sysfs, if sysfs is already existed when creating ramdump node,
return previous device handle but not create new sysfs.

Change-Id: I1aa020aa50ec487a1d3041f3d6e24366988f0289