drivers: usb: cdc_acm (next): reduce network buffer on full speed devices

[Sam3e2]

Is your enhancement proposal related to a problem? Please describe.
usbd_cdc_acm.h uses network buffers for each direction, 512 bytes each. I assume that 512 bytes are needed for high speed packets, only. Especially on small devices, 1 KB overhead per cdc acm instance can be significant.

Describe the solution you'd like
If the assumption is correct, the buffer size should be decreased to 64 bytes on full speed devices.