efi_loader: cleanup for tee backed variables

There's 2 variables in efi_get_next_variable_name() checking the size of
the variable name. Let's get rid of the reduntant definition and
simplitfy the code a bit.

Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>