Prepare the tekpower-dmm driver to be able to support various simple
serial port based DMMs.
Also, make a 'tekpower-tp4000zc' "first-class" driver which is currently
the only user of this generic driver.
Merge parts of the tekpower-dmm code (the chip of the TekPower
TP4000ZC seems to be an FS9721_LP3 too) and rework parts of the functions.
Adapt the tekpower-dmm and uni-t-dmm code accordingly.