irqchip/bcm-6345-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 bcm6345_l1_cpu" and this structure
ends in a flexible array:

struct bcm6345_l1_cpu {
	[...]
	u32	enable_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 safer.

Signed-off-by: Erick Archer <erick.archer@gmx.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
Link: https://lore.kernel.org/r/20240209181600.9472-1-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]