Bluetooth: btnxpuart: Enable flow control before checking boot signature

This enables flow control before checking for bootloader signature and
deciding whether FW download is needed or not. In case of V1 bootloader
chips w8987 and w8997, it is observed that if WLAN FW is downloaded first
and power save is enabled in wlan core, bootloader signatures are not
emitted by the BT core when the chip is put to sleep. As a result, the
driver skips FW download and subsequent HCI commands get timeout errors
in dmesg as shown below:

[  112.898867] Bluetooth: hci0: Opcode 0x c03 failed: -110
[  114.914865] Bluetooth: hci0: Setting baudrate failed (-110)
[  116.930856] Bluetooth: hci0: Setting wake-up method failed (-110)

By enabling the flow control, the host enables its RTS pin, and an
interrupt in chip's UART peripheral causes the bootloader to wake up,
enabling the bootloader signatures, which then helps in downloading
the bluetooth FW file.

This changes all instances of 0/1 for serdev_device_set_flow_control()
to false/true.

Signed-off-by: Neeraj Sanjay Kale <neeraj.sanjaykale@nxp.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>