net: dsa: Simplify 'dsa_tag_protocol_to_str()'

There is no point in preparing the module name in a buffer. The format
string can be passed diectly to 'request_module()'.

This axes a few lines of code and cleans a few things:
   - max len for a driver name is MODULE_NAME_LEN wich is ~ 60 chars,
     not 128. It would be down-sized in 'request_module()'
   - we should pass the total size of the buffer to 'snprintf()', not the
     size minus 1

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>