DC motor speed is too low while interfacing with L293D motor driver and ATmega8
Here's what the L293D spec says: -



In red I've extrapolated what this means for a 6V battery supply. So, in  a H bridge you can't really expect to put more than 3 or 4 volts onto your motor from a 6V supply. The device isn't modern and it isn't geared up for driving 6V motors from a 6V supply.

Also note the current drive capability of the device is 0.6A which is poor for driving any decent mechanical load.