fw_env: fix type of len

This variable is assigned by a size_t, and is printed that way, but is
incorrectly declared as an int.  Which means we get warnings:
fw_env.c: In function 'fw_setenv':
fw_env.c:409:5: warning: format '%zu' expects argument of type 'size_t',
	but argument 3 has type 'int' [-Wformat]

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>