staging: vt6655: Cleanup and rename function MACvSetLoopbackMode

Rename function MACvSetLoopbackMode to vt6655_mac_set_loopback_mode and
byLoopbackMode to loopback_mode to avoid CamelCase which is not accepted
by checkpatch.pl. Remove unnecessary declaration of function and made
function static. Change declaration of loopback_mode to shorten code and
remove unnecessary line break.

Signed-off-by: Philipp Hortmann <philipp.g.hortmann@gmail.com>
Link: https://lore.kernel.org/r/02ce61620e13eb7fd24833e59a288f52bf5b9730.1662890990.git.philipp.g.hortmann@gmail.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>