sparc32: mm: Fix argument checking in __srmmu_get_nocache()

The 'size' argument to __srmmu_get_nocache() is a number of bytes not
a shift value, so fix up the sanity checking to treat it properly.

Cc: "David S. Miller" <davem@davemloft.net>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Will Deacon <will@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>