FAT: remove cur_part_nr

FAT: remove cur_part_nr

A future patch will implement the more standard filesystem API
fat_set_blk_dev(). This API has no way to know which partition number
the partition represents. Equally, future DM rework will make the
concept of partition number harder to pass around.

So, simply remove cur_part_nr from fat.c; its only use is in a
diagnostic printf, and the context where it's printed should make it
obvious which partition is referred to anyway (since the partition ID
would come from the user command-line that caused it).

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Reviewed-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>