fs/fat: debug-print file read position during file_fat_read_at()

In order to make the debug print in file_fat_read_at() a tad more useful,
show the offset the file is being read at alongside the filename.

Suggested-by: Tero Kristo <t-kristo@ti.com>
Signed-off-by: Andreas Dannenberg <dannenberg@ti.com>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>