ARM: 6024/1: bcmring: fix missing down on semaphore in dma.c

Added missing down on the memMap->lock semaphore. Also fixed a return
statement so that we always exit with an up (i.e. early exit via return
is not allowed)

Signed-off-by: Leo Hao Chen <leochen@broadcom.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>