IB/qib: Default some module parameters optimally

Minimize the need for users to have to set module parameters to get
good performance.

The following two parameters are changed:
 - rcvhdrcnt to twice the rcvegrcnt
 - pcie_caps=0x51

The rcvhdrcnt at twice the egrcount allows the preemptive NAK code
during reception to function in 100% of the cases rather than a sender
jiffies-based timeout.

The pcie_caps default of 0x51 will set the proposed MaxPayload and
MaxReceiveReqest to 256 and 4096 respectively.  The capabilities on
the root complex will be used to limit those values.

Reviewed-by: Ram Vepa <ram.vepa@qlogic.com>
Signed-off-by: Mike Marciniszyn <mike.marciniszyn@qlogic.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>