yam: remove redundant null check on dev

yam_open has a redundant null check on null,  it will
never be called with dev == NULL. Remove this redundant check.
This also cleans up a smatch warning:

drivers/net/hamradio/yam.c:869 yam_open() warn: variable
  dereferenced before check 'dev' (see line 867)

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>