Bugfix/spitool exch parse txdata
* friendlier I2C "knock-knock" uses only read request
While many I2C slave devices have multiple indexed registers, some have
only one register and it's not indexed. For example, the I2C bus switch
TCA9548A has only a Control Register, attempting to index to "Reg[0]"
alters its contents to 0x00 disabling all subordinate buses. This patch
fixes that problem by simply trying to read something/anything from the
slave. This also helps coax out slaves with register files that start
from a higher index, i.e. the AT24CS0x, FLASH chips with a UUID that
appears as a 2nd I2C slave at (address+8), report their serial number at
Reg[80]-Reg[8F] and will NAK a read of Reg[0].
* modify get/set to prevent write of reg index if not specifed
* correctly parse exch txdata args, if any
* should always call the board's deselect
Approved-by: Gregory Nutt <gnutt@nuttx.org>
