mt76: use mt76_poll_msec routine in mt76pci_load_firmware()

Use mt76_poll_msec() in mt76pci_load_firmware to check if the firmware
has been started instead of explicitly poll MCU running register

Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>