staging: hikey9xx: hi6421-spmi-pmic: fix IRQ handler code

The conversion to regmap introduced a regression at the code
which reads from the IRQ register. Address that.

Fixes: fb02e3eb ("staging: hikey9xx: spmi driver: convert to regmap")
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/2eae710c333a8ee6f9e0a086c84115bc90a782ca.1611949675.git.mchehab+huawei@kernel.org


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>