fsl_pq_mdio: Fix compiler/sparse warnings (part 2)

This patch fixes following warnings:

fsl_pq_mdio.c:112:38: warning: cast adds address space to expression (<asn:2>)
fsl_pq_mdio.c:124:38: warning: cast adds address space to expression (<asn:2>)
fsl_pq_mdio.c:133:38: warning: cast adds address space to expression (<asn:2>)
fsl_pq_mdio.c:414:11: warning: cast adds address space to expression (<asn:2>)

Instead of adding __force all over the place, introduce convenient
fsl_pq_mdio_get_regs() call that does the ugly casting just once.

Signed-off-by: Anton Vorontsov <avorontsov@ru.mvista.com>
Signed-off-by: David S. Miller <davem@davemloft.net>