mtdparts: regroup calls to get_mtd_device_nm

The get_mtd_device_nm function is called in a couple places and the
string that is passed to it is not really used after the calls.

This patch regroups the calls to this function into a new function,
get_mtd_info.

Signed-off-by: Ben Gardiner <bengardiner@nanometrics.ca>
Acked-by: Stefan Roese <sr@denx.de>
CC: Wolfgang Denk <wd@denx.de>