i2c: sh_mobile: terminate DMA reads properly

DMA read requests could miss proper termination, so two more bytes would
have been read via PIO overwriting the end of the buffer with wrong
data. Make DMA stop handling more readable while we are here.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>