rockchip: spi: fix off-by-one in chunk size computation

The maximum transfer length (in a single transaction) for the Rockchip
SPI controller is 64Kframes (i.e. 0x10000 frames) of 8bit or 16bit
frames and is encoded as (num_frames - 1) in CTRLR1.  The existing
code subtracted the "minus 1" twice for a maximum transfer length of
0xffff (64K - 1) frames.

While this is not strictly an error (the existing code is correct, but
leads to a bit of head-scrating), fix this off-by-one situation.

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>