i2c: xgene-slimpro: Fix out-of-bounds bug in xgene_slimpro_i2c_xfer()

The data->block[0] variable comes from user and is a number between
0-255. Without proper check, the variable may be very large to cause
an out-of-bounds when performing memcpy in slimpro_i2c_blkwr.

Fix this bug by checking the value of writelen.

Fixes: f6505fba

 ("i2c: add SLIMpro I2C device driver on APM X-Gene platform")
Signed-off-by: Wei Chen <harperchen1110@gmail.com>
Cc: stable@vger.kernel.org
Reviewed-by: Andi Shyti <andi.shyti@kernel.org>
Signed-off-by: Wolfram Sang <wsa@kernel.org>