eficonfig: use u16_strsize() to get u16 string buffer size

Use u16_strsize() to simplify the u16 string buffer
size calculation.

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