IB/core: Refactor rate_show to use ib_port_attr_to_rate()

IB/core: Refactor rate_show to use ib_port_attr_to_rate()

Update sysfs rate_show() to rely on ib_port_attr_to_speed_info() for
converting IB port speed and width attributes to data rate and speed
string.

Signed-off-by: Or Har-Toov <ohartoov@nvidia.com>
Reviewed-by: Maher Sanalla <msanalla@nvidia.com>
Reviewed-by: Mark Bloch <mbloch@nvidia.com>
Signed-off-by: Edward Srouji <edwards@nvidia.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>