net: tftp: use IS_ENABLED(CONFIG_NET_TFTP_VARS) instead of #if

Nothing inside this block depends on NET_TFTP_VARS to be set to parse
correctly. Switch to C if() in preparation for adding code before
this (to avoid a declaration-after-statement warning).

Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
[trini: Update to cover CONFIG_TFTP_PORT case as well]
Signed-off-by: Tom Rini <trini@konsulko.com>