net/mlx5e: Fix module EEPROM query

When querying the module EEPROM, there was a misusage of the 'offset'
variable vs the 'query.offset' field.
Fix that by always using 'offset' and assigning its value to
'query.offset' right before the mcia register read call.

While at it, the cross-pages read size adjustment was changed to be more
intuitive.

Fixes: e19b0a34 ("net/mlx5: Refactor module EEPROM query")
Reported-by: Wang Yugui <wangyugui@e16-tech.com>
Signed-off-by: Gal Pressman <gal@nvidia.com>
Reviewed-by: Maxim Mikityanskiy <maximmi@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>