Update MicroLCDpro.cpp

Per : http://digistump.com/wiki/digispark/tutorials/oled

Fix: reduce the number of bytes per transfer block in MicroLCDPro.cpp/LCD_SSD1306::clear around line #630 (width »= 4; make j go to 16 couple of lines further down) I've forked the repo and will provide patches soonish … 

Can not get my USB running just now to test - coding this as suggested: Edit l;ne 633.  Not sure it is a pure "Fix" as there are other anomalies and on the UNO device this all works perfectly.