TFTP: rename STATE_RRQ to STATE_SEND_RRQ

With the upcoming TFTP server implementation, requests can be either
outgoing or incoming, 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>