efi_loader: Fix buffer underflow

If the array index 'i' < 128, the 'codepage' array is accessed using
[-128...-1] in efi_unicode_collation.c:262. This can lead to a buffer
overflow.

    Negative index in efi_unicode_collation.c:262.

The index of the 'codepage' array should be c - 0x80 instead of i - 0x80.

Fixes: 0bc4b0da ("efi_loader: EFI_UNICODE_COLLATION_PROTOCOL")
Signed-off-by: Mikhail Ilin <ilin.mikhail.ol@gmail.com>
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>