Give the mx25l device sole ownership of the name /dev/flash/spi* instead of
trying to use disk_add_alias() to make spi* an alias for mx25l*. It turns
out disk_add_alias() works for partitions, but not slices, and that's hard
to fix.

This change is, in effect, a partial revert of r344526.

The mips world relies on the existence of flashmap names formatted as
/dev/flash/spi0s.name, whereas pretty much nothing relies on at45d devices
using the /dev/spi* names (because until recently the at45d driver didn't
even work reliably). So this change makes mx25l devices the sole owner of
the /dev/flash/spi* namespace, which actually makes some sense because it is
a SpiFlash(tm) device, so flash/spi isn't a horrible name.

Reported by: Mori Hiroki <yamori813@yahoo.co.jp>