ethtool: rename local variable max -> curr

ethtool_set_channels() validates the config against driver's max
settings. It retrieves the current config and stores it in a
variable called max. This was okay when only max settings were
accessed but we will soon want to access current settings as
well, so calling the entire structure max makes the code less
readable.

While at it drop unnecessary parenthesis.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Quentin Monnet <quentin.monnet@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>