On an AMD chromebook, where the same I2C bus is shared by both Raydium
touchscreen and a trackpad device, it is observed that interleaving of
I2C messages when `raydium_i2c_read_message()` is called leads to the
Raydium touch IC reporting incorrect information. This is the sequence
that was observed to result in the above issue:

* I2C write to Raydium device for RM_CMD_BANK_SWITCH
* I2C write to trackpad device
* I2C read from trackpad device
* I2C write to Raydium device for setting address
* I2C read from Raydium device >>>> This provides incorrect
  information

This change adds a new helper function `raydium_i2c_xfer()` that
performs I2C transactions to the Raydium device. It uses the register
address to decide if RM_CMD_BANK_SWITCH header needs to be sent to the
device (i.e. if register address is greater than 255, then bank switch
header is sent before the rest of the transaction). Additionally, it
ensures that all the I2C operations performed as part of
`raydium_i2c_xfer()` are done as a single i2c_transfer. This
guarantees that no other transactions are initiated to any other
device on the same bus in between. Additionally,
`raydium_i2c_{send|read}*` functions are refactored to use this new
helper function.

Verified with the patch across multiple reboots (>100) that the
information reported by the Raydium  touchscreen device during probe
is correct.
Signed-off-by: Furquan Shaikh <furquan@google.com>
Link: https://lore.kernel.org/r/20200821024006.3399663-1-furquan@google.comSigned-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>