bhnd(4): Fix a SPROM identification regression introduced in r315866

In r315866, we introduced a direct read of the 8-bit sromrev field from the
memory mapped SPROM/OTP device. On OTP devices that require 16-bit access
alignment, this read fails, preventing identification of the SPROM layout.

So, let's perform an aligned read of the combined 16-bit sromrev/crc field
instead.

Approved by: adrian (mentor, implicit)