cmd_fat.c: fix build warning

Commit cfda5aea
(cmd_fat: use common get_device_and_partition function)
introduced a warning:
cmd_fat.c: In function 'do_fat_fswrite':
cmd_fat.c:178:8: warning: unused variable 'ep' [-Wunused-variable]

Fix it.

Signed-off-by: Anatolij Gustschin <agust@denx.de>
Reported-by: Albert ARIBAUD <albert.u.boot@aribaud.net>
Tested-by: Albert ARIBAUD <albert.u.boot@aribaud.net>