mmc: core: fix reading the bkops status from card

The bkops status is indicated by the bit 0 and 1 of the
246th byte of the ext_csd register.
The current code doesn't ignore the rest of the byte.

Fix this by extracting the bit 0 and 1 only
for the current bkops urgency.

The exception level is defined by the least significant
nibble of 54th byte in the ext_csd register. The current
code doesn't ignore the rest of the byte.

Fix this by extracting the nibble(LSB) for exception status.

Change-Id: Ic90fe26a676ae7dd2063e17bc3771db83605f4dc
Signed-off-by: Asutosh Das <asutoshd@codeaurora.org>