fsi: Lock mutex for master device registration

Because master device registration may cause hub master scans, or
user scans may begin before device registration has ended, so the
master scan lock must be held while registering the device.

Signed-off-by: Eddie James <eajames@linux.ibm.com>
Link: https://lore.kernel.org/r/20230809180814.151984-3-eajames@linux.ibm.com


Signed-off-by: Joel Stanley <joel@jms.id.au>