a4m072: change 'display' command implementation

This patch changes implementation of 'display' command for a4m072
that uses 7-segment LED display as customer requested:

a) The "display" command shall _not_ turn on the decimal point.

b) Exception: "display ." shall turn on (only) the decimal point.

c) Hex digits (0-9, A-F, a-f) shall be displayed as usual.

d) Letters U, P, Y, L, S, T, H shall be displayed as usual (lower
   case letters identical to upper case letters)

e) 'I' (and 'i') shall be displayed like '1'

f) 'O' (and 'o') shall be displayed like '0'

g) all other Characters shall be displayed like ' ' (all segments off).

Signed-off-by: Ilya Yanok <yanok@emcraft.com>