common: update: Fix TFTP return value handling

common: update: Fix TFTP return value handling

The net_loop() returns 1 on success, but update_load() returns 0 on
success. Do not assign rv which is the return value of update_load()
to net_loop(), instead assign net_loop() return value to a temporary
variable and then update rv only if the temporary variable is negative.
This way the update_load() now correctly returns 0 on tftp success and
1 only on failure.

Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>