mfd: omap-usb-tll: Use struct_size to allocate tll

Commit 16c2004d ("mfd: omap-usb-tll: Allocate driver data at once")
changed the memory allocation of 'tll' to consolidate it into a single
allocation, introducing an incorrect size calculation.

In particular, the allocation for the array of pointers was converted
into a single-pointer allocation.

The memory allocation used to occur in two steps:

tll = devm_kzalloc(dev, sizeof(struct usbtll_omap), GFP_KERNEL);
tll->ch_clk = devm_kzalloc(dev, sizeof(struct clk *) * tll->nch,
                           GFP_KERNEL);

And it turned that into the following allocation:

tll = devm_kzalloc(dev, sizeof(*tll) + sizeof(tll->ch_clk[nch]),
                   GFP_KERNEL);

sizeof(tll->ch_clk[nch]) returns the size of a single pointer instead of
the expected nch pointers.

This bug went unnoticed because the allocation size was small enough to
fit within the minimum size of a memory allocation for this particular
case [1].

The complete allocation can still be done at once with the struct_size
macro, which comes in handy for structures with a trailing flexible
array.

Fix the memory allocation to obtain the original size again.

Link: https://lore.kernel.org/all/202406261121.2FFD65647@keescook/ [1]
Fixes: 16c2004d ("mfd: omap-usb-tll: Allocate driver data at once")
Reviewed-by: Kees Cook <kees@kernel.org>
Signed-off-by: Javier Carrasco <javier.carrasco.cruz@gmail.com>
Fixes: commit 16c2004d ("mfd: omap-usb-tll: Allocate driver data at once")
Link: https://lore.kernel.org/r/20240626-omap-usb-tll-counted_by-v2-1-4bedf20d1b51@gmail.comSigned-off-by: Lee Jones <lee@kernel.org>

drivers/mfd/omap-usb-tll.c

@@ -230,8 +230,7 @@ static int usbtll_omap_probe(struct platform_device *pdev)
 		break;
 	}
 
-	tll = devm_kzalloc(dev, sizeof(*tll) + sizeof(tll->ch_clk[nch]),
-			   GFP_KERNEL);
+	tll = devm_kzalloc(dev, struct_size(tll, ch_clk, nch), GFP_KERNEL);
 	if (!tll) {
 		pm_runtime_put_sync(dev);
 		pm_runtime_disable(dev);