cpupower: Change the var type of the 'monitor' subcommand display mode

There is a type 'enum operation_mode_e' contains the display modes of
the 'monitor' subcommand. This type isn't used though, instead the
variable 'mode' is of a simple 'int' type.
Change 'mode' variable type from 'int' to 'enum operation_mode_e' in
order to improve compiler type checking.
Built and tested this with different monitor cmdline params. Everything
works as expected, that is nothing changed and no regressions encountered.

Signed-off-by: Roman Storozhenko <romeusmeister@gmail.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>