soc/cores/uart: use reset_less on accumulator, reg, bitcount to reduce.

soc/cores/uart: use reset_less on accumulator, reg, bitcount to reduce.

This reduces logic a bit. It does not make large difference on usual design with
only 1 UART, but is interesting on designs with hundreds of UARTs used to "document"
FPGA boards :) (similar to https://github.com/enjoy-digital/camlink_4k/blob/master/ios_stream.py)