bnxt_re/libbnxtre: Add support for Variable WQE on BCM576xx controllers

Variable size WQE means that each send Work Queue Entry to
HW can use different WQE sizes as opposed to the static WQE
size on the current devices. Set variable WQE mode for BCM576xx
devices. Depth of the Queue will be a multiple of slot which
is 16 bytes. The number of slots should be a multiple of 256
as per the HW requirement.
Initialize the Software shadow queue to hold requests equal to
the number of slots.

Reviewed by: ssaxena
Differential Revision: https://reviews.freebsd.org/D54525
MFC after: 3 days