fw_env: fix reading NVMEM device's "compatible" value

Call to fread() was changed to check for return value. The problem is it
can't be checked for returning 1 (as it is) to determine success.

We call fread() with buffer size as "size" argument. Reading any
"compatible" value shorter than buffer size will result in returning 0
even on success.

Modify code to use fstat() to determine expected read length.

This fixes regression that broke using fw_env with NVMEM devices.

Fixes: c059a22b ("tools: env: fw_env: Fix unused-result warning")
Cc: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>