* tx/rx: Rename wrap_addr to start_addr for clarity
* Fix RX logic, when advancing the tail pointer, not retrieving the stored RX buffer start address but the default value instead
* Fix TX logic, when advancing the head pointer for next transmission, not retrieving the stored TX buffer end address but the default value instead
* Rename SPI-related const's to match the datasheet: RERXDATA→RRXDATA, WEGPDATA→WGPDATA
* Remove useless const's for SRAM default addresses for TX buffer
* Simplify code
* lib.rs: fix transmission status checking (line 176)
* lib.rs: make RAW_FRAME_LENGTH_MAX the same for both RX and TX, for the purpose of setting MAMXFL in controller, as well as setting MTU for smoltcp device
* smoltcp_phy.rs: fix missing MTU definition
Harry Ho