net: ethernet: oa_tc6: Handle failure of spi_setup

net: ethernet: oa_tc6: Handle failure of spi_setup

There is no guarantee that spi_setup succeed, so properly handle
the error case.

Fixes: aa58bec064ab ("net: ethernet: oa_tc6: implement register write operation")
Signed-off-by: Stefan Wahren <wahrenst@gmx.net>
Cc: stable@kernel.org
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Link: https://patch.msgid.link/20250827115341.34608-2-wahrenst@gmx.net
Signed-off-by: Jakub Kicinski <kuba@kernel.org>