dropbear: restore performance by disabling mips16

Disable MIPS16 to prevent it negatively affecting performance.
Observed was a increase of connection delay from ~6 to ~11 seconds
and a reduction of scp speed from 1.1MB/s to 710kB/s on brcm63xx.

Fixes #15209.

Signed-off-by: Jonas Gorski <jogo@openwrt.org>

SVN-Revision: 42250