net/tftp.c: cosmetic: fix pointer syntax issues

This removes the following checkpatch issues:
 - ERROR: "foo * bar" should be "foo *bar"
 - ERROR: "(foo*)" should be "(foo *)"

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