norflash: Clear status register before sending commands

norflash: Clear status register before sending commands

The status register bits remain until explicitly cleared, which means
that a command can be incorrectly considered to have generated an error -
for example, after reset the status register contents may be unknown or
if a previous command had failed.

This patch clears the status register before beginning any command to
be sure that the status register only represents information about the
current operation.

Change-Id: I9e98110ee24179937215461c00b6543a3467b350
Signed-off-by: Roberto Vargas <[email protected]>