nvme-pci: remove an unneeded variable initialization

Variable "n" will be assigned once kstrtoint() succeeds, otherwise it
will not be referred because kstrtoint() will return an error which
means go out from this function.

Signed-off-by: Minwoo Im <minwoo.im.dev@gmail.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>