mtd: rawnand: nandsim: Clean error handling

Many function calls are done this way:

        if ((retval = func()) != 0)
	        return retval;

while we expect in the kernel function calls like:

        retval = func();
	if (retval)
	        return retval;

Apply this change where possible and also use "ret" instead of
"retval" in ns_init_module for consistency, as it is only used in this
function.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20200525085851.17682-5-miquel.raynal@bootlin.com