usb: bluetooth: Hardcode USB EP sizes

The existing calculation of the BULK_EP size can sometimes result in a
bulk EP size that is not spec-compliant. The accepted BULK_EP sizes are
8, 16, 32 or 64 bytes.

Additionally, the INT_EP size is always set to the max EP size since
BT_BUF_RX_SIZE is always greater than that (min. 73 in Kconfig).

This commit sets the BULK and INT endpoint sizes to their maximum
supported values.

Signed-off-by: Jonathan Rico <jonathan.rico@nordicsemi.no>