mctp i2c: initialise event handler read bytes

mctp i2c: initialise event handler read bytes

Set a 0xff value for i2c reads of an mctp-i2c device. Otherwise reads
will return "val" from the i2c bus driver. For i2c-aspeed and
i2c-npcm7xx that is a stack uninitialised u8.

Tested with "i2ctransfer -y 1 r10@0x34" where 0x34 is a mctp-i2c
instance, now it returns all 0xff.

Fixes: f5b8abf9fc3d ("mctp i2c: MCTP I2C binding driver")
Signed-off-by: Matt Johnston <matt@codeconstruct.com.au>
Link: https://patch.msgid.link/20260113-mctp-read-fix-v1-1-70c4b59c741c@codeconstruct.com.au
Signed-off-by: Jakub Kicinski <kuba@kernel.org>