coresight: tmc: set buffer size to 32MB on SW USB mode

We are using memory as buffer for SW USB mode. Too small
buffer could cause packet drop issue. Always set ETR
buffer to 32MB on SW USB mode to resolve this issue.

Change-Id: I222152f15c5cc61141b9a9eb18c1b2bb19714174
Signed-off-by: Tingwei Zhang <tingwei@codeaurora.org>