efi_loader: fix struct efi_input_key

The UEFI specification defines filed UnicodeChar as CHAR16. We use
u16 for CHAR16 throughout our code. The change fixes the following errors:

    lib/efi_loader/initrddump.c: In function ‘efi_input’:
    lib/efi_loader/initrddump.c:218:38: warning:
    comparison is always false due to limited range of data type
    [-Wtype-limits]
      218 | if (key.unicode_char >= 0xD800 && key.unicode_char <= 0xDBFF)
          |                      ^~
    lib/efi_loader/initrddump.c:218:68: warning:
    comparison is always true due to limited range of data type
    [-Wtype-limits]
      218 | if (key.unicode_char >= 0xD800 && key.unicode_char <= 0xDBFF)
          |                                                    ^~

Fixes: 867a6ac8 ("efi: Add start-up library code")
Reported-by: Marek Vasut <marex@denx.de>
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>