[SCSI] advansys: Remove library-style callback routines

Convert adv_isr_callback, adv_async_callback and asc_isr_callback into
direct calls.  Remove the unused asc_exe_callback.

Signed-off-by: Matthew Wilcox <matthew@wil.cx>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>