coresight: tmc: Fix TMC mode read in tmc_read_unprepare_etb()

coresight: tmc: Fix TMC mode read in tmc_read_unprepare_etb()

Reading TMC mode register without proper coresight power
management can lead to exceptions like the one in the call
trace below in tmc_read_unprepare_etb() when the trace data
is read after the sink is disabled. So fix this by having
a check for coresight sysfs mode before reading TMC mode
management register in tmc_read_unprepare_etb() similar to
tmc_read_prepare_etb().

  SError Interrupt on CPU6, code 0xbe000411 -- SError
  pstate: 80400089 (Nzcv daIf +PAN -UAO)
  pc : tmc_read_unprepare_etb+0x74/0x108
  lr : tmc_read_unprepare_etb+0x54/0x108
  sp : ffffff80d9507c30
  x29: ffffff80d9507c30 x28: ffffff80b3569a0c
  x27: 0000000000000000 x26: 00000000000a0001
  x25: ffffff80cbae9550 x24: 0000000000000010
  x23: ffffffd07296b0f0 x22: ffffffd0109ee028
  x21: 0000000000000000 x20: ffffff80d19e70e0
  x19: ffffff80d19e7080 x18: 0000000000000000
  x17: 0000000000000000 x16: 0000000000000000
  x15: 0000000000000000 x14: 0000000000000000
  x13: 0000000000000000 x12: 0000000000000000
  x11: 0000000000000000 x10: dfffffd000000001
  x9 : 0000000000000000 x8 : 0000000000000002
  x7 : ffffffd071d0fe78 x6 : 0000000000000000
  x5 : 0000000000000080 x4 : 0000000000000001
  x3 : ffffffd071d0fe98 x2 : 0000000000000000
  x1 : 0000000000000004 x0 : 0000000000000001
  Kernel panic - not syncing: Asynchronous SError Interrupt

Fixes: 4525412a5046 ("coresight: tmc: making prepare/unprepare functions generic")
Reported-by: Mike Leach <mike.leach@linaro.org>
Signed-off-by: Sai Prakash Ranjan <saiprakash.ranjan@codeaurora.org>
Tested-by: Mike Leach <mike.leach@linaro.org>
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Link: https://lore.kernel.org/r/20200716175746.3338735-14-mathieu.poirier@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>