* Fixes the lack of setup time delay of an fixed-byte command that immediately follows an N-byte command.
* Extra 50ns will be inserted in-between fixed-byte commands but should be acceptable.
* 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