scsi: aic7xxx: Fix build using bare-metal toolchain

Bare-metal toolchains don't define __linux__, so aic7xxx build with
bare-metal toolchain is broken. This driver codebase used to be partially
shared with FreeBSD, but these days there is no point in keeping the
compatibility around. So let's just drop FreeBSD related code and get rid
of __linux__ checking in order to fix the build using bare-metal
toolchains.

Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>