usb: dwc3: refactor gadget endpoint count calculation

usb: dwc3: refactor gadget endpoint count calculation

- DWC_USB3_NUM indicates the number of Device mode single directional
  endpoints, including OUT and IN endpoint 0.

- DWC_USB3_NUM_IN_EPS indicates the maximum number of Device mode IN
  endpoints active at any time, including control endpoint 0.

It's possible to configure RTL such that DWC_USB3_NUM_EPS is equal to
DWC_USB3_NUM_IN_EPS.

dwc3-core calculates the number of OUT endpoints as DWC_USB3_NUM minus
DWC_USB3_NUM_IN_EPS. If RTL has been configured with DWC_USB3_NUM_IN_EPS
equal to DWC_USB3_NUM then dwc3-core will calculate the number of OUT
endpoints as zero.

For example a from dwc3_core_num_eps() shows:
[    1.565000]  /usb0@f01d0000: found 8 IN and 0 OUT endpoints

This patch refactors the endpoint calculation down to one variable
dwc->num_eps taking care to maintain the current mapping of endpoints for
fixed FPGA configurations as described in Table 4-7 of version 2.60a of the
DWC USB3 databook.

The endpoint mapping will then be EP-OUT, EP-IN etc, up to DWC_USB3_NUM.
If DWC_USB3_NUM is odd then OUT will take the extra endpoint.

Suggested-by: Felipe Balbi <balbi@kernel.org>
Signed-off-by: Bryan O'Donoghue <pure.logic@nexus-software.ie>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>