TFTP: replace "server" with "remote" in local variable names

With the upcoming TFTP server implementation, the remote node can be
either a client or a server, so avoid ambiguities.

Signed-off-by: Luca Ceresoli <luca.ceresoli@comelit.it>
Cc: Wolfgang Denk <wd@denx.de>
Acked-by: Detlev Zundel <dzu@denx.de>