ice: move ice_devlink_flash_update and merge with ice_flash_pldm_image

The ice_devlink_flash_update function performs a few upfront checks and
then calls ice_flash_pldm_image.

Most if these checks make more sense in the context of code within
ice_flash_pldm_image. Merge ice_devlink_flash_update and
ice_flash_pldm_image into one function, placing it in ice_fw_update.c

Since this is still the entry point for devlink, call the function
ice_devlink_flash_update instead of ice_flash_pldm_image. This leaves a
single function which handles the devlink parameters and then initiates
a PLDM update.

With this change, the ice_devlink_flash_update function in
ice_fw_update.c becomes the main entry point for flash update. It
elimintes some unnecessary boiler plate code between the two previous
functions. The ultimate motivation for this is that it eases supporting
a dry run with the PLDM library in a future change.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Gurucharan G <gurucharanx.g@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>