Use maps to convert for strings to internal constants and vice versa
in aiptek's sysfs attribute methods instead of open-coding it. This
results in smaller code that is also easier to maintain.

[Rene: fix a typo - stylys instead of stylus]
Signed-off-by: NRene van Paassen <rene.vanpaassen@gmail.com>
Signed-off-by: NDmitry Torokhov <dtor@mail.ru>