cmd: mvebu/bubt: Correctly propagate failure during tftp transport

net_loop() returns signed int type and negative value represents error.
tftp_read_file() returns unsigned size_t type and zero value represents
error. Casting signed negative value to unsigned size_t type cause losing
information about error and bubt thinks that no error happened, and
continue erasing SPI-NOR which cause malfunction device.

Fix this issue by correctly propagating failure during tftp transport.

With this change when there is no eth link, bubt does not erase SPI-NOR
anymore.

  => bubt
  Burning U-Boot image "flash-image.bin" from "tftp" to "spi"
  ethernet@30000 Waiting for PHY auto negotiation to complete......... TIMEOUT !
  ethernet@30000: No link.
  Error: Failed to read file flash-image.bin from tftp

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Stefan Roese <sr@denx.de>