staging: vt6655: Introduce `idx` variable

Add a local variable to store `MISCFIFO_SYNDATA_IDX` offset.
This change helps in shortening the lines in `rf.c` that
are deemed too long by checkpatch.pl.

Reviewed-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Karolina Drobnik <karolinadrobnik@gmail.com>
Link: https://lore.kernel.org/r/596ae9bc980379860b5b1785b3ecc88e18121103.1636729147.git.karolinadrobnik@gmail.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>