net: sfp: change st_mutex locking

Change st_mutex's use within SFP such that it only protects the various
state members, as it was originally supposed to, and isn't held while
making various calls outside the driver.

Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>