net: xilinx: axienet: Fix kernel-doc warnings for missing return descriptions

Add missing "Return:" sections to kernel-doc comments for four functions:
- axienet_calc_cr()
- axienet_device_reset()
- axienet_free_tx_chain()
- axienet_dim_coalesce_count_rx()

Also standardize the return documentation format by replacing inline
"Returns" text with proper "Return:" tags as per kernel documentation
guidelines.

Fixes below kernel-doc warnings:
- Warning: No description found for return value of 'axienet_calc_cr'
- Warning: No description found for return value of 'axienet_device_reset'
- Warning: No description found for return value of 'axienet_free_tx_chain'
- Warning: No description found for return value of
'axienet_dim_coalesce_count_rx'

Signed-off-by: Suraj Gupta <suraj.gupta2@amd.com>
Link: https://patch.msgid.link/20250919103754.434711-1-suraj.gupta2@amd.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>