9p: Small cleanup in <net/9p/9p.h>

There are two small cleanups in this patch:
 - p9_errstr2errno was declared twice - remove one declaration.
 - A uint8_t type was mixed in, change it to u8 to match
with the rest of the type names and remove dependency.

Cc: Eric Van Hensbergen <ericvh@gmail.com>
Cc: Ron Minnich <rminnich@sandia.gov>
Cc: Latchesar Ionkov <lucho@ionkov.net>
Cc: Pekka Enberg <penberg@cs.helsinki.fi>
Signed-off-by: Sasha Levin <levinsasha928@gmail.com>
Signed-off-by: Eric Van Hensbergen <ericvh@gmail.com>