platform: msm: Rectify buffer overread in QPNP-PWM driver

Remove the bug in the buffer-read loop that reads beyond
the size of the input buffer.

Change-Id: Iecb420d2a5f0159b7b2eb0b4a5a4feb66c427d19
Signed-off-by: Jay Chokshi <jchokshi@codeaurora.org>