net: dsa: mv88e6xxx: Fix uninitialised err value

The err value in mv88e6xxx_region_atu_snapshot is now potentially
uninitialised on return. Initialise err as 0.

Fixes: ada5c322 ("net: dsa: mv88e6xxx: Add FID map cache")
Signed-off-by: Aryan Srivastava <aryan.srivastava@alliedtelesis.co.nz>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/20241009212319.1045176-1-aryan.srivastava@alliedtelesis.co.nz


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