power_spi.c: Rewrite pmic_reg function

The pmic_spi_free function isn't ever used, and as the frameworks stand
today, cannot be, so remove it.  Integrate the probe function into
pmic_reg as it's not really a "probe" today.  Finally, add an err label
for the common failure cases.

Cc: Lukasz Majewski <l.majewski@samsung.com>
Cc: Przemyslaw Marczak <p.marczak@samsung.com>
Cc: Stefano Babic <sbabic@denx.de>
Signed-off-by: Tom Rini <trini@ti.com>
Acked-by: Stefano Babic <sbabic@denx.de>