i2c: i801: make p2sb_spinlock a mutex

p2sb_spinlock is used in i801_add_tco_spt() only, and in process context
only. Therefore a mutex is sufficient, and we can make the definition
local to i801_add_tco_spt().

Reviewed-by: Jean Delvare <jdelvare@suse.de>
Reviewed-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>