mtd: nand: ecc-hamming: Clarify the logic around rp17

mtd: nand: ecc-hamming: Clarify the logic around rp17

This code has been written in 2008 and is fine, but in order to keep
robots happy, I think it's time to change a little bit this code just
to clarify the different possible values of eccsize_mult. Indeed, this
variable may only take the value 1 or 2 because step_size, in the case
of the software Hamming ECC engine may only be 256 or 512. Depending
on the value of eccsize_mult, an extra rp17 variable is set, or not
and triggers the following warning:

     smatch warnings:
     ecc_sw_hamming_calculate() error: uninitialized symbol 'rp17'.

As highlighted by Dan Carpenter, if the only possible values for
eccsize_mult are 1 and 2, then the code is fine, but "it's hard to
tell just from looking".

So instead of shifting step_size, let's use a ternary condition to
assign to eccsize_mult the only two possible values and clarify the
driver's logic.

Now that the situation is clarified for humans, ensure rp17 is
initialized to 0 to keep compilers and robots silent as well.

Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20201030172333.28390-1-miquel.raynal@bootlin.com