[media] tuner-xc2028: use request_firmware_nowait()

Change the firmware logic to use request_firmware_nowait(), and
to preserve the loaded firmwares in memory, to reduce the risk
of troubles with buggy userspace apps.

With this change, while the firmware is being loaded, the driver
will return -EAGAIN to any calls. If, for some reason, firmware
failed to be loaded from userspace, it will return -ENODEV.

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>