staging: pi433: rf69.c style fix - spaces required around

This patch fixes the following checkpatch.pl errors:

ERROR: spaces required around that '+=' (ctx:WxV)
ERROR: spaces required around that '=' (ctx:VxV)
ERROR: spaces required around that '<' (ctx:VxV)

in rf69.c file as requested by TODO file.

Additionally some style warnings remain valid here and could be fixed by
another patch.

Signed-off-by: Marcin Ciupak <marcin.s.ciupak@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>