Drivers: char: misc: 'misc_deregister()' changed the 'mutex_unlock' logic upon an error

This change improves code readability & is less error-prone.
For example: case adding more error paths one should remember to call 'mutex_unlock'

Signed-off-by: Elad Wexler <elad.wexler@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>