net: encx24j600: Fix invalid logic in reading of MISTAT register

A loop for reading MISTAT register continues while regmap_read() fails
and (mistat & BUSY), but if regmap_read() fails a value of mistat is
undefined.

The patch proposes to check for BUSY flag only when regmap_read()
succeed. Compile test only.

Found by Linux Verification Center (linuxtesting.org) with SVACE.

Fixes: d70e5326

 ("net: Microchip encx24j600 driver")
Signed-off-by: Valentina Goncharenko <goncharenko.vp@ispras.ru>
Reviewed-by: Pavan Chebbi <pavan.chebbi@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>