src/mbr: fix r_mbr_switch_set_boot_partition() error paths
src/mbr: fix r_mbr_switch_set_boot_partition() error paths

res variable will be set to true by the previous call of
get_raw_partition_entry(). Thus only setting the error and going to
'out' will result in accidentally returning TRUE in case of failures of
lseek() or write() call.

Signed-off-by: Enrico Joerns <a2e5f936c390f36494e46e71bcfbe365bb1e8c49@pengutronix.de>

jluebbe/rauc,rauc/rauc,ejoerns/rauc,ejoerns/rauc,jluebbe/rauc,ejoerns/rauc,rauc/rauc,jluebbe/rauc,rauc/rauc