Any write or erase command will be ignored unless this bit is set, so
make sure the WEL bit is set before exiting write_enable().
The WEL bit is set in response to the Write Enable command, but prior to
this change, the WEL bit status was not being checked.  This would
result in occasional erase failures.

This addresses two failure modes associated with the reading of the BUSY
status bit:  the case when the BUSY bit is not yet set on entering the
function, and the case where it is briefly cleared but immediately
reset by the chip.  The latter is not really documented behavior but has
been observed on Winbond W25Q128JV