net: bgmac: Fix an erroneous kfree() in bgmac_remove()

'bgmac' is part of a managed resource allocated with bgmac_alloc(). It
should not be freed explicitly.

Remove the erroneous kfree() from the .remove() function.

Fixes: 34a5102c

 ("net: bgmac: allocate struct bgmac just once & don't copy it")
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Link: https://lore.kernel.org/r/a026153108dd21239036a032b95c25b5cece253b.1655153616.git.christophe.jaillet@wanadoo.fr
Signed-off-by: Jakub Kicinski <kuba@kernel.org>