phy: cadence-torrent: Check return value on register read

cdns_torrent_dp_set_power_state() does not consider that ret might be
overwritten. Add return value check of regmap_read_poll_timeout() after
register read in cdns_torrent_dp_set_power_state().

Fixes: 5b16a790

 ("phy: cadence-torrent: Reorder few functions to remove function declarations")
Signed-off-by: Ma Ke <make24@iscas.ac.cn>
Reviewed-by: Roger Quadros <rogerq@kernel.org>
Link: https://lore.kernel.org/r/20240702032042.3993031-1-make24@iscas.ac.cn
Signed-off-by: Vinod Koul <vkoul@kernel.org>