common/command.c: revert changes from commit 199adb60

commit 199adb60

 "common/misc: sparse
fixes" broke the help command trying to fix the sparse error
"command.c:44:38: error: bad constant expression".

As Henrik points out, the fix was bad because the commit used
CONFIG_SYS_MAXARGS whereas the code intended to use the maximum
number of commands (not arguments to a command).

Revert command.c changes to the original code as asked by Wolfgang.

Reported-by: Henrik Nordström <henrik@henriknordstrom.net>
Signed-off-by: Anatolij Gustschin <agust@denx.de>