cmd_mmc: fix bootpart-resize maxarg to 4

sub-command 'bootpart-resize' check for argc == 4,
it will retrun CMD_RET_FAILURE when argc value not matched.

but bootpart-resize's maxarg is 3, which means you never execute
this sub-command successfully.

fix it by change bootpart-resize maxarg to 4.

Signed-off-by: wally.yeh <wally.yeh@atrustcorp.com>
Cc: Wolfgang Denk <wd@denx.de>
Cc: Pierre Aubert <p.aubert@staubli.com>
Acked-by: Pantelis Antoniou <panto@antoniou-consulting.com>