net: stmmac: Delete txtimer in suspend()

When running v5.5 with a rootfs on NFS, memory abort may happen in
the system resume stage:
 Unable to handle kernel paging request at virtual address dead00000000012a
 [dead00000000012a] address between user and kernel address ranges
 pc : run_timer_softirq+0x334/0x3d8
 lr : run_timer_softirq+0x244/0x3d8
 x1 : ffff800011cafe80 x0 : dead000000000122
 Call trace:
  run_timer_softirq+0x334/0x3d8
  efi_header_end+0x114/0x234
  irq_exit+0xd0/0xd8
  __handle_domain_irq+0x60/0xb0
  gic_handle_irq+0x58/0xa8
  el1_irq+0xb8/0x180
  arch_cpu_idle+0x10/0x18
  do_idle+0x1d8/0x2b0
  cpu_startup_entry+0x24/0x40
  secondary_start_kernel+0x1b4/0x208
 Code: f9000693 a9400660 f9000020 b4000040 (f9000401)
 ---[ end trace bb83ceeb4c482071 ]---
 Kernel panic - not syncing: Fatal exception in interrupt
 SMP: stopping secondary CPUs
 SMP: failed to stop secondary CPUs 2-3
 Kernel Offset: disabled
 CPU features: 0x00002,2300aa30
 Memory Limit: none
 ---[ end Kernel panic - not syncing: Fatal exception in interrupt ]---

It's found that stmmac_xmit() and stmmac_resume() sometimes might
run concurrently, possibly resulting in a race condition between
mod_timer() and setup_timer(), being called by stmmac_xmit() and
stmmac_resume() respectively.

Since the resume() runs setup_timer() every time, it'd be safer to
have del_timer_sync() in the suspend() as the counterpart.

Signed-off-by: Nicolin Chen <nicoleotsuka@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>