mtd: rawnand: placate "$VARIABLE is used uninitialized" warnings

The compiler is not smart enough to notice that it's impossible for
them to be actually used uninitialized.  Which exact variables trip
here varies depending on random surrounding code; none triggered in
6.1-rc1 but 6.1-rc2 fails on three of these five, despite variables
declared in the very same line having identical flow.

Signed-off-by: Adam Borowski <kilobyte@angband.pl>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20221024092026.42123-1-kilobyte@angband.pl