nfsroot: set tcp as the default transport protocol

UDP is disabled by default in commit b24ee6c6 ("NFS: allow
deprecation of NFS UDP protocol"), but the default mount options
is still udp, change it to tcp to avoid the "Unsupported transport
protocol udp" error if no protocol is specified when mount nfs.

Fixes: b24ee6c6 ("NFS: allow deprecation of NFS UDP protocol")
Signed-off-by: Liwei Song <liwei.song@windriver.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>