ARM: assabet: fix bogus warning in get_assabet_scr (again)

Russell fixed this bogus warning before in 2f3eca8b "Shut up gcc
warning in assabet.c", but apparently gcc has become smarter (or dumber)
since 2005, and the same warning came up again.

This uses the uninitialized_var() macro to convince gcc that the
variable is actually being initialized. 100 times in fact.

Without this patch, building assabet_defconfig results in:

arch/arm/mach-sa1100/assabet.c: In function 'fixup_assabet':
arch/arm/mach-sa1100/assabet.c:397:6: warning: 'scr' may be used uninitialized in this function [-Wuninitialized]
arch/arm/mach-sa1100/assabet.c:389:16: note: 'scr' was declared here

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Russell King <rmk+kernel@arm.linux.org.uk>