Use "bx lr", or "pop {lr}", which do proper mode switching
between thumb and arm modes. A plain "mov pc, lr" does not switch
from thumb mode to arm mode (while in arm mode, it does switch
mode for a thumb caller).
This is normally not an issue, as CONFIG_THUMB only is enabled if
the C compiler defaults to thumb; but stick to patterns that can
do mode switching if needed, for consistency.
Signed-off-by: Martin Storsjö <martin@martin.st>