net: lan78xx: fix "softirq work is pending" error

Disable BH around the call to napi_schedule() to avoid following
error:
NOHZ tick-stop error: local softirq work is pending, handler #08!!!

Fixes: ec4c7e12

 ("lan78xx: Introduce NAPI polling support")
Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Link: https://lore.kernel.org/r/20240226110820.2113584-1-o.rempel@pengutronix.de
Signed-off-by: Jakub Kicinski <kuba@kernel.org>