drm/tinydrm: Remove spi debug buffer dumping

The SPI event tracing can dump the buffer now so no need for this.
Remove the debug print from tinydrm_spi_transfer() since this info can be
gleaned from the trace event.

Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190719155916.62465-5-noralf@tronnes.org