hw/esp.c does not properly deal with TEST_UNIT_READY in NetBSD/sparc

The NetBSD ncr53c9x.c driver does a TEST_UNIT_READY command with SELATN but dma disabled sometimes (early during bus enumeration). This is fine, as the command will not produce nor consume any data, and works on real hardware.

However, the qemu emulation does not allow this (for reasons I don't understand).

The change below fixes the problem.