efi: libstub: fix type confusion for load_options_size

Even though it is unlikely to ever make a difference, let's use u32
consistently for the size of the load_options provided by the firmware
(aka the command line)

While at it, do some general cleanup too: use efi_char16_t, avoid using
options_chars in places where it really means options_size, etc.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>