regmap: Quit the read-loop based on position of file-offset

Some implementation's of the 'cat' command pass the
absolute position of the file-offset. In the current
implementation of 'regmap_data_read_file' this leads
to dumping all the registers from offset 0.

Avoid this by quitting the read loop if the file-offset
requested is lower than actual read-offset.

Change-Id: I7c6c31958e92f16387bf4cb5de7fbf4572fcce21
Signed-off-by: Anirudh Ghayal <aghayal@codeaurora.org>