speakup: document the usage of enum values

The cursor tracking modes enum, the edge enum, and the read_all commands
enum should be used as such in the source code, to make it more readable
and make gcc catch missing values in switches.

Also, some values of enums are coupled with others, we at least need to
document these.

Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Link: https://lore.kernel.org/r/20201102095945.ap4pdff2dn47hijh@function
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>