test: efi_selftest: Do not force serial# setting

As part of the EFI self test we set and check the serial# variable.
However, we should not be forcing this setting.  In the case where we
are allowed to change the variable it will change, and we will pass the
test.  In the case where we cannot change it, force may or may not be
allowed, depending on further environment restrictions.  Drop the -f
flag here as we do not need it.

Cc: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Tom Rini <trini@konsulko.com>