spi: mpc512x: minor prep before feature change

mechanical edits before changing functionality, to reduce diffs
- rename variables to later tell remaining TX and RX apart
- use size_t for the current TX and RX length
  (for better compatibility with the min() macro)
- remove unused members from the master data (sysclk, eofbyte)
- silence a checkpatch warning (braces around single statement)

Signed-off-by: Gerhard Sittig <gsi@denx.de>
Signed-off-by: Mark Brown <broonie@linaro.org>