cmd_mmc: fix arg parsing for setdsr subcmd

The handler do_setdsr receives only the dsr parameter,
the action is parsed before.

Error was introduced when restructuring the mmc command
implementation in commit 1fd93c6e.

Reported-by: Michael Krummsdorf <Michael.Krummsdorf@tq-group.com>
Signed-off-by: Markus Niebel <Markus.Niebel@tq-group.com>