net/tftp.c: cosmetic: fix brace issues

This removes the following checkpatch issues:
 - WARNING: braces {} are not necessary for single statement blocks
 - WARNING: braces {} are not necessary for any arm of this statement

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