coresight: Fix run time warnings while reusing ETR buffer

Fix the below warning by avoding calls to tmc_etr_enable_hw,
if we are reusing the ETR buffer for multiple sources in sysfs mode.

echo 1 > /sys/bus/coresight/devices/tmc_etr0/enable_sink
echo 1 > /sys/bus/coresight/devices/ete1/enable_source
echo 1 > /sys/bus/coresight/devices/ete2/enable_source
[  166.918290] ------------[ cut here ]------------
[  166.922905] WARNING: CPU: 4 PID: 2288 at
drivers/hwtracing/coresight/coresight-tmc-etr.c:1037
tmc_etr_enable_hw+0xb0/0xc8
[  166.933862] Modules linked in:
[  166.936911] CPU: 4 PID: 2288 Comm: bash Not tainted 6.5.0-rc7 #132
[  166.943084] Hardware name: Marvell CN106XX board (DT)
[  166.948127] pstate: 834000c9 (Nzcv daIF +PAN -UAO +TCO +DIT -SSBS
BTYPE=--)
[  166.955083] pc : tmc_etr_enable_hw+0xb0/0xc8
[  166.959345] lr : tmc_enable_etr_sink+0x134/0x210
snip..
  167.038545] Call trace:
[  167.040982]  tmc_etr_enable_hw+0xb0/0xc8
[  167.044897]  tmc_enable_etr_sink+0x134/0x210
[  167.049160]  coresight_enable_path+0x160/0x278
[  167.053596]  coresight_enable+0xd4/0x298
[  167.057510]  enable_source_store+0x54/0xa0
[  167.061598]  dev_attr_store+0x20/0x40
[  167.065254]  sysfs_kf_write+0x4c/0x68
[  167.068909]  kernfs_fop_write_iter+0x128/0x200
[  167.073345]  vfs_write+0x1ac/0x2f8
[  167.076739]  ksys_write+0x74/0x110
[  167.080132]  __arm64_sys_write+0x24/0x38
[  167.084045]  invoke_syscall.constprop.0+0x58/0xf8
[  167.088744]  do_el0_svc+0x60/0x160
[  167.092137]  el0_svc+0x40/0x170
[  167.095273]  el0t_64_sync_handler+0x100/0x130
[  167.099621]  el0t_64_sync+0x190/0x198
[  167.103277] ---[ end trace 0000000000000000 ]---
-bash: echo: write error: Device or resource busy

Fixes: 296b01fd ("coresight: Refactor out buffer allocation function for ETR")
Signed-off-by: Linu Cherian <lcherian@marvell.com>
Reviewed-by: James Clark <james.clark@arm.com>
Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Link: https://lore.kernel.org/r/20230823042948.12879-1-lcherian@marvell.com