net/mlx5e: Fetch WQE: reuse code and enforce typing

There are multiple functions mlx5{e,i}_*_fetch_wqe that contain the same
code, that is repeated, because they operate on different SQ struct
types. mlx5e_sq_fetch_wqe also returns void *, instead of the concrete
WQE type.

This commit generalizes the fetch WQE operation by putting this code
into a single function. To simplify calls of the generic function in
concrete use cases, macros are provided that substitute the right WQE
size and cast the return type.

Before this patch, fetch_wqe used to calculate pi itself, but the value
was often known to the caller. This calculation is moved outside to
eliminate this unnecessary step and prepare for the fill_frag_edge
refactoring in the next patch.

Signed-off-by: Maxim Mikityanskiy <maximmi@mellanox.com>
Reviewed-by: Tariq Toukan <tariqt@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>