[SCSI] qla2xxx: Remove unnecessary spinlock primitive - mbx_reg_lock.

Since, mailbox commands are executed in a synchronous
manner, there is no need to have a separate spinlock
primitive to protect data/register access shared by callers.

Signed-off-by: Seokmann Ju <seokmann.ju@qlogic.com>
Signed-off-by: Andrew Vasquez <andrew.vasquez@qlogic.com>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>