rust: phy: use to_result for error handling

rust: phy: use to_result for error handling

Simplifies error handling by replacing the manual check
of the return value with the `to_result` helper.

Signed-off-by: Onur Özkan <work@onurozkan.dev>
Reviewed-by: Elle Rhumsaa <elle@weathered-steel.dev>
Reviewed-by: Trevor Gross <tmgross@umich.edu>
Link: https://patch.msgid.link/20250821091235.800-1-work@onurozkan.dev
Signed-off-by: Jakub Kicinski <kuba@kernel.org>