cmd: efidebug: fix a failure of "boot rm" sub-command

There is a wrong usage of utf8_utf16_strncpy() in "boot rm" command, and
then it will end up with a failure of this command due to a wrong
value of an interim variable ("var_name16").

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>