irqchip/irq-bcm7038-l1: Prefer struct_size over open coded arithmetic

This is an effort to get rid of all multiplications from allocation
functions in order to prevent integer overflows.

The cpu variable is a pointer to "struct bcm7038_l1_cpu" and this structure
ends in a flexible array:

struct bcm7038_l1_cpu {
	void __iomem	*map_base;
	u32		mask_cache[];
};

The preferred way in the kernel is to use the struct_size() helper to
do the arithmetic instead of the argument "size + count * size" in the
kzalloc() function.

This way, the code is more readable and more safer.

Signed-off-by: Erick Archer <erick.archer@gmx.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/20240209183128.10273-1-erick.archer@gmx.com
Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#open-coded-arithmetic-in-allocator-arguments [1]
Link: https://github.com/KSPP/linux/issues/162 [2]