net/tftp.c: cosmetic: do not use assignment in if condition

net/tftp.c: cosmetic: do not use assignment in if condition

This removes the following checkpatch issue:
 - ERROR: do not use assignment in if condition.

There is one such error left:

  ERROR: do not use assignment in if condition
  #239: FILE: tftp.c:239:
  +		if (!ProhibitMcast
  +		 && (Bitmap = malloc(Mapsize))
  +		 && eth_get_dev()->mcast) {

which would require an additional nested if to be fixed, resulting in longer
and less readable code.

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