mfd: wm8350-core: Supply description wm8350_reg_{un}lock args

Kerneldoc syntax is used, but not complete.  Descriptions required.

Prevents warnings like:

 drivers/mfd/wm8350-core.c:136: warning: Function parameter or member 'wm8350' not described in 'wm8350_reg_lock'
 drivers/mfd/wm8350-core.c:165: warning: Function parameter or member 'wm8350' not described in 'wm8350_reg_unlock'

Cc: patches@opensource.cirrus.com
Signed-off-by: Lee Jones <lee.jones@linaro.org>