tmc-etr: Correct condition for SW USB mode when setup sysfs buf

Correct condition for SW USB mode when setup sysfs buf to fix the issue
that the allocated buffer size is always 32M for memory mode with sw_usb
is enabled.

Change-Id: Id545ecfa21378f49ddeac4c3c3f4fe51f744e69b
Signed-off-by: Mao Jinlong <jinlmao@codeaurora.org>