drivers: disk: nvme: fix warnings

There are warnings generated within NVMe. `prp` is a `void *` which
is 32bits wide on 32bit systems. This adds a cast to first cast it
to a `uintptr_t` and then casts it to a `uint64_t` to supress the
warning.

This also fix an issue where `int n_prp` is defined under a case
statement. This adds the { } around the block underneath it.

Signed-off-by: Ryan McClelland <ryanmcclelland@meta.com>