cmd: ubi: print load size after establishing volume size

When using static volumes, the file size stored in the volume is
determined at runtime. Currently the ubi command prints the file
size specified on the console, which leads to a rather confusing
series of messages:
  # ubi read ${fdt_addr_r} testvol
  Read 0 bytes from volume testvol to 82000000
  No size specified -> Using max size (179924992)

Make sure to print the actual size read in any case:
  # ubi read ${fdt_addr_r} testvol
  No size specified -> Using max size (179924992)
  Read 179924992 bytes from volume testvol to 82000000

Signed-off-by: Stefan Agner <stefan.agner@toradex.com>