mtd: nand: lpc32xx_slc: Remove unneeded NULL check on 'rc'

devm_ioremap_resource() does a NULL check on the 'rc' argument, so
remove the unneeded manual NULL check.

While at it, place the 'rc' assignment just before
devm_ioremap_resource() to improve readability.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Acked-by: Marek Vasut <marek.vasut@gmail.com>
Acked-by: Vladimir Zapolskiy <vz@mleia.com>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>