msm: rtb: Fix buffer corruption issue

Consider the case of a nentries==8 and 3 cpus.
Numbers in parenthesis are the equivalent location in the circular buffer.
CPU:   Index0:  Index1: Index2: Index3:
0      0        3       6       9(1)
1      1        4       7       10(2)
2      2        5       8(0)

The current design is only appropriate for the case where
nentries % nrcpus == 0.

Fix this issue by incrementing the index by (nentries % nrcpus)
each time circular buffer wraps around.

CPU:   Index0:  Index1: Index2:
0      0        3       6+2==8(0)
1      1        4       7+2==9(1)
2      2        5       8+2==10(2)

Change-Id: I4f96eb4c971cc18357e145dabcf4272e466dcda2
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>