selftests: firmware: remove use of non-standard diff -Z option

"diff -Z" is used to trim the trailing whitespace when comparing the
loaded firmware file with the source firmware file. However, per the
comment in the source code, -Z should not be necessary. In testing, the
input and output files are identical.

Additionally, -Z is not a standard option and is not available in
environments such as busybox. When -Z is not supported, diff fails with
a usage error, which is suppressed, but then causes read_firmwares() to
exit with a false failure message.

Signed-off-by: Dan Rue <dan.rue@linaro.org>
Acked-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Shuah Khan <shuah@kernel.org>