send_adb_cuda() would assume the returned value of splhigh() is the macintr

send_adb_cuda() would assume the returned value of splhigh() is the macintr
interrupt enable register; this used to be true more than 20 years ago but
the interrupt code has changed a lot since, and it is nevertheless bad
practice from md drivers to know to much about spl*() return values.

The check should become a "spl >= IPL_TTY", but management of the
adb_polling variable is good enough to need this at all.