[PATCH] i2c: make better use of IDR in i2c-core

This patch uses the already existing IDR mechanism to simplify and
improve the i2c_get_adapter function in i2c-core.

Signed-off-by: Mark M. Hoffman <mhoffman@lightlink.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>