fat: remove FAT_FIRST_ENT macro

fat: remove FAT_FIRST_ENT macro

The comment edited in this patch was the only reference to the
FAT_FIRST_ENT macro, which is not used anymore.  Moreover, the commented
line of code does not compile with the current code.

Since the FAT_FIRST_ENT macro checks the FAT variant in a way that the
patch series changes, I removed it, and instead wrote a clear
explanation of what was checked.

I verified that the changed comment is correct according to Microsoft
FAT spec, search for "BPB_Media" in the following references:

1. Microsoft FAT specification 2005
(http://read.pudn.com/downloads77/ebook/294884/FAT32%20Spec%20%28SDA%20Contribution%29.pdf).
Search for 'volume label'.
2. Microsoft Extensible Firmware Initiative, FAT32 File System Specification
(https://staff.washington.edu/dittrich/misc/fatgen103.pdf).
Search for 'volume label'.

Link: http://lkml.kernel.org/r/1544990640-11604-2-git-send-email-carmeli.tamir@gmail.com
Signed-off-by: Carmeli Tamir <carmeli.tamir@gmail.com>
Acked-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Reviewed-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Cc: Bart Van Assche <bvanassche@acm.org>
Cc: Johannes Thumshirn <jthumshirn@suse.de>
Cc: Martin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>