[tosa_mlir_translator] Fix Variable Op serialization

Tosa variable op interface has changed.
the attribute "type" used to specify variable type,
but now it specifies element type.
Changed to use tosa::getVariableType(VariableOp) to get
type of variable.

Signed-off-by: Tai Ly <tai.ly@arm.com>
Change-Id: Idc7adc546c1f1eafacc0a96167bdb32929c7cb27