drm/amdgpu: Fixes to the AMDGPU EEPROM driver

* When reading from the EEPROM device, there is no
  device limitation on the number of bytes
  read--they're simply sequenced out. Thus, read
  the whole data requested in one go.

* When writing to the EEPROM device, there is a
  256-byte page limit to write to before having to
  generate a STOP on the bus, as well as the
  address written to mustn't cross over the page
  boundary (it actually rolls over). Maximize the
  data written to per bus acquisition.

* Return the number of bytes read/written, or -errno.

* Add kernel doc.

Cc: Jean Delvare <jdelvare@suse.de>
Cc: Alexander Deucher <Alexander.Deucher@amd.com>
Cc: Andrey Grodzovsky <Andrey.Grodzovsky@amd.com>
Cc: Lijo Lazar <Lijo.Lazar@amd.com>
Cc: Stanley Yang <Stanley.Yang@amd.com>
Cc: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Luben Tuikov <luben.tuikov@amd.com>
Acked-by: Alexander Deucher <Alexander.Deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>