net: ethtool: store netdev in a temp variable in ethnl_default_set_doit()

For ease of review of the next patch store the dev pointer
on the stack, instead of referring to req_info.dev every time.

No functional changes.

Reviewed-by: Michael Chan <michael.chan@broadcom.com>
Link: https://patch.msgid.link/20250119020518.1962249-3-kuba@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>