efi_loader: EFI_FILE_PROTOCOL.Write() check args

Check the parameters passed to Write():

* cannot write to directories (UEFI SCT 2017, 5.7.3.5.15)
* cannot write to file opened read only (UEFI SCT 2017, 5.7.3.5.16)

Add missing comments.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>