sgi-xp: make some symbols static in xpnet.c

sgi-xp: make some symbols static in xpnet.c

Fix the following sparse warning:

drivers/misc/sgi-xp/xpnet.c:99:19: warning: symbol 'xpnet_device' was
not declared. Should it be static?
drivers/misc/sgi-xp/xpnet.c:134:22: warning: symbol 'xpnet_dbg_name' was
not declared. Should it be static?
drivers/misc/sgi-xp/xpnet.c:138:15: warning: symbol 'xpnet_dbg_subname'
was not declared. Should it be static?
drivers/misc/sgi-xp/xpnet.c:143:15: warning: symbol 'xpnet' was not
declared. Should it be static?

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Jason Yan <yanaijie@huawei.com>
Link: https://lore.kernel.org/r/20200410063618.27143-1-yanaijie@huawei.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>