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

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

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

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

Signed-off-by: Rahul Kumar <rk0006818@gmail.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>