mmc: Fix incorrect handling of 'read' & 'write' commands

If a malformed 'read' or 'write' command is issued, the Sandbox U-Boot
can crash because the command-handling code does no error checking on
the number of provided arguments.

This change makes the mmc 'erase', 'read' and 'write' commands only
function if the proper number of arguments are supplied.

Also puts the else assignment at the beginning fo the if() statement
to shortens the generated code.  This removes an unnecessary jump from
the generated code.

Signed-off-by: Taylor Hutt <thutt@chromium.org>
Signed-off-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Andy Fleming <afleming@freescale.com>