sfc: Convert to use ERR_CAST()

As opposed to open-code, using the ERR_CAST macro clearly indicates that
this is a pointer to an error value and a type conversion was performed.

Signed-off-by: Shen Lichuan <shenlichuan@vivo.com>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Reviewed-by: Martin Habets <habetsm.xilinx@gmail.com>
Reviewed-by: Edward Cree <ecree.xilinx@gmail.com>
Link: https://patch.msgid.link/20240829021253.3066-1-shenlichuan@vivo.com


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