i2c: pmcmsp: return message count on master_xfer success

[ Upstream commit de9a8634f1cb4560a35696d472cc7f1383d9b866 ]

Returning zero is wrong in this case.

Signed-off-by: Peter Rosin <peda@axentia.se>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>
Fixes: 1b144df1 ("i2c: New PMC MSP71xx TWI bus driver")
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>