efi/libstub: Cast away type warning in use of max()

commit 61d130f2 upstream.

Avoid a type mismatch warning in max() by switching to max_t() and
providing the type explicitly.

Fixes: 3cb4a482 ("efi/libstub: fix efi_random_alloc() ...")
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>