watchdog: f71808e_wdt: Fix magic close handling

[ Upstream commit 7bd3e7b743956afbec30fb525bc3c5e22e3d475c ]

Watchdog close is "expected" when any byte is 'V' not just the last one.
Writing "V" to the device fails because the last byte is the end of string.

$ echo V > /dev/watchdog
f71808e_wdt: Unexpected close, not stopping watchdog!

Signed-off-by: Igor Pylypiv <igor.pylypiv@gmail.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>