net: airoha: Compute number of descriptors according to reserved memory size

In order to not exceed the reserved memory size for hwfd buffers,
compute the number of hwfd buffers/descriptors according to the
reserved memory size and the size of each hwfd buffer (2KB).

Fixes: 3a1ce9e3 ("net: airoha: Add the capability to allocate hwfd buffers via reserved-memory")
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Link: https://patch.msgid.link/20250619-airoha-hw-num-desc-v4-1-49600a9b319a@kernel.org


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