efi: Remove redundant null pointer checks in efi_debugfs_init()

Since the debugfs_create_dir() never returns a null pointer, checking
the return value for a null pointer is redundant, and using IS_ERR is
safe enough.

Signed-off-by: Li Zetao <lizetao1@huawei.com>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>