ipg: remove some internal comments

This removes some now useless comments that were added when the driver was
developed out-of-tree.

Signed-off-by: Pekka Enberg <penberg@cs.helsinki.fi>
Cc: Francois Romieu <romieu@fr.zoreil.com>
Cc: Sorbica Shieh <sorbica@icplus.com.tw>
Cc: Jesse Huang <jesse@icplus.com.tw>