mtd: cfi: use struct_size() helper for cfiq allocation

mtd: cfi: use struct_size() helper for cfiq allocation

Documentation/process/deprecated.rst recommends against performing
dynamic size calculations in the arguments of memory allocator
function due to the risk of overflow. Such calculations can
wrap around and result in a smaller allocation than what the caller
was expecting.

Replace the size calculation in cfiq allocation with struct_size()
helper to make the code clearer and handle the overflows correctly.

Signed-off-by: Gopi Krishna Menon <krishnagopi487@gmail.com>
Reviewed-by: Vignesh Raghavendra <vigneshr@ti.com>
link: https://lore.kernel.org/linux-kernel-mentees/20250922071137.900508-1-rk0006818@gmail.com/T/#u
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>