net/tftp.c: cosmetic: fix whitespace issues

This removes the following checkpatch issues:
 - ERROR: space prohibited before that close parenthesis ')'
 - ERROR: space required after that ';' (ctx:BxV)
 - ERROR: space required after that ',' (ctx:VxV)
 - ERROR: space required after that ';' (ctx:VxV)
 - ERROR: spaces required around that '<<=' (ctx:VxV)
 - ERROR: spaces required around that '<' (ctx:VxV)
 - ERROR: spaces required around that '=' (ctx:VxV)
 - ERROR: spaces required around that '+=' (ctx:VxV)
 - ERROR: spaces required around that '=' (ctx:VxW)
 - WARNING: please, no spaces at the start of a line
 - WARNING: space prohibited between function name and open parenthesis '('

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