devlink: move request_firmware out of driver

All drivers which implement the devlink flash update support, with the
exception of netdevsim, use either request_firmware or
request_firmware_direct to locate the firmware file. Rather than having
each driver do this separately as part of its .flash_update
implementation, perform the request_firmware within net/core/devlink.c

Replace the file_name parameter in the struct devlink_flash_update_params
with a pointer to the fw object.

Use request_firmware rather than request_firmware_direct. Although most
Linux distributions today do not have the fallback mechanism
implemented, only about half the drivers used the _direct request, as
compared to the generic request_firmware. In the event that
a distribution does support the fallback mechanism, the devlink flash
update ought to be able to use it to provide the firmware contents. For
distributions which do not support the fallback userspace mechanism,
there should be essentially no difference between request_firmware and
request_firmware_direct.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Acked-by: Shannon Nelson <snelson@pensando.io>
Acked-by: Vasundhara Volam <vasundhara-v.volam@broadcom.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>