Fix constant conversion warnings in ppbus(4)

[dim]

Summary

With clang 3.9.0, compiling ppbus results in the following warnings:

sys/dev/ppbus/ppb_1284.c:296:46: error: implicit conversion from 'int' to 'char' changes value from 144 to -112 [-Werror,-Wconstant-conversion]
        if ((error = do_peripheral_wait(bus, SELECT | nBUSY, 0))) {
                     ~~~~~~~~~~~~~~~~~~      ~~~~~~~^~~~~~~
sys/dev/ppbus/ppb_1284.c:785:48: error: implicit conversion from 'int' to 'char' changes value from 240 to -16 [-Werror,-Wconstant-conversion]
                if (do_1284_wait(bus, nACK | SELECT | PERROR | nBUSY,
                    ~~~~~~~~~~~~      ~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~
sys/dev/ppbus/ppb_1284.c:786:29: error: implicit conversion from 'int' to 'char' changes value from 240 to -16 [-Werror,-Wconstant-conversion]
                                        nACK | SELECT | PERROR | nBUSY)) {
                                        ~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~

This is because nBUSY is 0x80, so the char argument is wrapped to a
negative value. I have tried to fix this in a minimal fashion, by using
uint8_t in just a few places.

Test Plan

Is there anybody who still has functioning parallel port hardware? :)

[emaste]

LGTM