ap51-flash: Define IPPORT_TFTP where it is used

ap51-flash: Define IPPORT_TFTP where it is used

Defining IPPORT_TFTP on the pre-processor command line leads to the
following error for glibc-based toolchains:

mipsel-linux-gnu-gcc -Os -pipe -mno-branch-likely -mips32r2 -mtune=24kc
-fno-caller-saves -mips16 -minterlink-mips16
-I/local/users/fainelli/openwrt/trunk/staging_dir/target-mipsel-linux-gnu_glibc/usr/include
-I/local/users/fainelli/openwrt/trunk/staging_dir/target-mipsel-linux-gnu_glibc/include
-I/opt/toolchains/stbgcc-4.8-1.5/usr/include
-I/opt/toolchains/stbgcc-4.8-1.5/include  -Wall -Werror -W -g3
-std=gnu99 -Os -fno-strict-aliasing -DLINUX
-DREVISION_VERSION=\"2016-10-16\" -DFLASH_FROM_FILE -DNO_LIBPCAP
-D_GNU_SOURCE -UIPPORT_TFTP -DIPPORT_TFTP=69 -MD -c flash.c -o flash.o
<command-line>:0:13: error: expected identifier before numeric constant

Instead, move this where this gets used, in proto.c

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>