scsi: esas2r: Replace semaphore fs_api_semaphore with mutex

The semaphore 'fs_api_semaphore' is used as a simple mutex, so it should
be written as one. Semaphores are going away in the future.

Signed-off-by: Binoy Jayan <binoy.jayan@linaro.org>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>