As of now, tg3_power_down_prepare always ends with success, but
the error handling code from former tg3_set_power_state call is still here.

This code became unreachable in commit c866b7ea ("tg3: Do not use
legacy PCI power management").

Remove (now unreachable) error handling code for simplification and change
tg3_power_down_prepare to a void function as its result is no more checked.
Signed-off-by: Nikita Kiryushin <kiryushin@ancud.ru>
Reviewed-by: Michael Chan <michael.chan@broadcom.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://lore.kernel.org/r/20240401191418.361747-1-kiryushin@ancud.ruSigned-off-by: Paolo Abeni <pabeni@redhat.com>