ShellPkg/TftpDynamicCommand: Return proper status

ShellPkg/TftpDynamicCommand: Return proper status

Tftp command always returned "SHELL_NOT_FOUND" which is treated as an
error by callers. Add missing line to clean the ShellStatus on
successful operation. If operation has failed, return the error status
if available.

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=1487

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Vladimir Olovyannikov <vladimir.olovyannikov@broadcom.com>
Reviewed-by: Songpeng Li <songpeng.li@intel.com>
Reviewed-by: Jaben Carsey <jaben.carsey@intel.com>