staging: r8188eu: Add _enter_critical_mutex() error handling

_enter_critical_mutex() is a simple call to mutex_lock_interruptible(),
but there is no error handling code for it.

The patch removes wrapper _enter_critical_mutex() and
adds error handling for mutex_lock_interruptible().

Found by Linux Driver Verification project (linuxtesting.org).

Signed-off-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>