arm64: debug: fix mdscr.ss check when enabling debug exceptions

When we take an exception at EL1, we only want to enable debug
exceptions if we're not currently stepping, otherwise we can easily get
stuck in a loop stepping into interrupt handlers.

Unfortunately, the current code tests the wrong bit in the mdscr, so fix
that.

Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>