net: tftp: silence a subscript above array bounds compile time warning

For strncpy() select a minimal string length of destination and source
strings, here DEFAULT_NAME_LEN is preferable to MAX_LEN.

Due to the NUL-terminated contents of default_string the change is
a noop, however it removes a compilation warning if SH2/3/4 platform
specific strncpy() function is used:

  In file included from include/linux/string.h:21:0,
                   from include/common.h:28,
                   from net/tftp.c:9:

  net/tftp.c: In function 'tftp_start':
  arch/sh/include/asm/string.h:52:42: warning: array subscript is above array bounds [-Warray-bounds]
     : "0" (__dest), "1" (__src), "r" (__src+__n)

Signed-off-by: Vladimir Zapolskiy <vz@mleia.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>