Remove unneeded variable ret in function sisusb_set_default_mode.

Change return type of sisusb_set_default_mode from int to void
as it never fails.

Issue identified by coccicheck

Signed-off-by: Hariprasad Kelam <hariprasad.kelam@gmail.com>
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changes in v2: Change return type of sisusb_set_default_mode from int to
void as it never fails

changes in v3: Update changelog
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Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>