powerpc/5200: in lpbfifo, flag DMA irqs as enabled after requesting them

This patch avoids unbalanced enable/disable messages for the DMA
interrupts when running the 5200 platform SCLPC/BestComm driver in DMA
mode.

Signed-off-by: Roman Fietze <roman.fietze@telemotive.de>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>