cxgbe(4): Rename rx_c_chan to rx_chan.

It is the equivalent of tx_chan but for receive so rx_chan is a better
name. Initialize both using helper functions and make sure both are
displayed in the sysctl MIB.

Sponsored by: Chelsio Communications

(cherry picked from commit 480ff89c67b25113515018cdcd13179229b4a0d3)