media: tvaudio: improve error handling

The error handling logic at tvaudio is broken on several ways,
as it doesn't really check right when an error occurs.

Change it to return the proper error code from read/write
routines and fix the errors on reads.

Shuts up the following warnings:
	drivers/media/i2c/tvaudio.c:222 chip_read() error: uninitialized symbol 'buffer'.
	drivers/media/i2c/tvaudio.c:223 chip_read() error: uninitialized symbol 'buffer'.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>