mfd: macsmc: Initialize mutex

mfd: macsmc: Initialize mutex

Initialize struct apple_smc's mutex in apple_smc_probe(). Using the
mutex uninitialized surprisingly resulted only in occasional NULL
pointer dereferences in apple_smc_read() calls from the probe()
functions of sub devices.

Cc: stable@vger.kernel.org
Fixes: e038d985c9823 ("mfd: Add Apple Silicon System Management Controller")
Signed-off-by: Janne Grunau <j@jannau.net>
Reviewed-by: Sven Peter <sven@kernel.org>
Reviewed-by: Neal Gompa <neal@gompa.dev>
Link: https://patch.msgid.link/20251231-macsmc-mutex_init-v2-1-5818c9dc9b29@jannau.net
Signed-off-by: Lee Jones <lee@kernel.org>