msm: sysmon: Send only strlen(tx_buf) instead of ARRAY_SIZE(tx_buf)

The data in tx_buf is always a NUL-terminated string. Instead of
sending the full 50 byte buffer to the destination subsystem,
send only the length of the string.

Change-Id: Ib0f3a14f4c5cb76c4f1027c8b6018d07bb487fa5
Signed-off-by: Matt Wagantall <mattw@codeaurora.org>