rockchip: mkimage: remove (left-over) assignment w/o effect [coverity]

An assignment (of a value to itself) was left over (after removing and
addition from the line) from moving the common padding code into
rkcommon_vrec_header.

This change removes this to avoid a spurious warning in static code
analysis (i.e. Coverity).

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reported-by: Coverity (CID: 161418)
Reviewed-by: Tom Rini <trini@konsulko.com>