Staging: dt3155: fix 50Hz configuration

According to the header file, dt3155_io.h, the 50/60 Hz configuration
is controlled by a bit in the I2C CSR2 register (bit 2).  The function
dt3155_init_isr actually reads the I2C CONFIG register into the global
I2C_CSR union variable then modifies the bit.  It then does a write
to the I2C CONFIG register with the global I2C_CONFIG union variable
which is not even set with a value anywhere in the driver.

My guess is 50Hz operation doesn't even work as-is.

Fix this by actually reading and writing the correct register with
the correct value.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Cc: Simon Horman <horms@verge.net.au>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>