Remove FAT function prototypes from the cm5200 firmware update code, and
include the relevant headers instead.

This exposes the fact that the custom prototyoe for do_fat_read() in
this file was incorrect. Rather than simply fixing the call-site, replace
do_fat_read() with fat_exists(). This removes the only use of
do_fat_read() outside of the FAT code.

Signed-off-by: Stephen Warren <swarren@wwwdotorg.org>