mmc: do not change mode when accessing a boot partition

Accessing the boot partition had been error prone with HS200 and HS400 and
was disabled. The driver first switched to a lesser mode and then switched
the partition access. It was mostly due to a bad handling of the switch and
has been fixed, so let's remove this 'feature'

Signed-off-by: Jean-Jacques Hiblot <jjhiblot@ti.com>