staging: exfat: Rename variable 'Day' to 'day'

Fix checkpatch warning: Avoid CamelCase
Change all occurrences of identifier "Day" to "day"

Signed-off-by: Pragat Pandya <pragat.pandya@gmail.com>
Link: https://lore.kernel.org/r/20200210183558.11836-4-pragat.pandya@gmail.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>