i2c: altera: cleanup spinlock

Protect altr_i2c_int_enable() by the mutex and remove unneeded spinlock.

Signed-off-by: Atsushi Nemoto <atsushi.nemoto@sord.co.jp>
Reviewed-by: Thor Thayer <thor.thayer@linux.intel.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>