mmc: vub300: Use common code in __download_offload_pseudocode()

Add a jump target so that a specific string copy operation is stored
only once at the end of this function implementation.
Replace two calls of the function "strncpy" by goto statements.

This issue was detected by using the Coccinelle software.

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>